Post WWII Commercially Manufactured M1 Carbines (U.S.A.)

PLAINFIELD MACHINE CO., INC.
Middlesex, New Jersey

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Part IPart IIPart IIIPart IVPart V
History
of
Plainfield Machine Co.
1951-1978
Serial Numbers
&
Dates of Manufacture
Models
&
Flier/Manual
Downloads
Receivers
&
Markings
Parts
&
Markings

Part IV

Receivers & Markings

All of the receivers used by Plainfield Machine, including those used for Millville Ordnance and H&S, were manufactured by investment casting. The design of the receivers changed over time as did the markings. The information presented in this section will cover these changes chronologically and will include serial number sequences, when known.

The quality of the investment castings and their machining varied at different times. Plainfield is known to have employed machinist apprentices in the production of their firearms. Some of these apprentices were still in high school. Generally, the casting and machining of the majority of receivers was well done. Especially the very early ones machined by owner William Haas.

Section I

Serial Numbers B-Z

(1962-1965)


Plainfield's First .30 Caliber M1 Carbines: Serial Numbers beginning with B

As with the serial numbers the first carbines with the markings of Plainfield Machine Company (PMC) were a continuation of the carbines with the H&S markings and the last carbines used by Millville Ordnance. The receivers and barrels have a number of very distinctive characteristics that evolved into what is found on the later Plainfield M1 Carbines and continued on with the carbines manufactured by Iver Johnson, Federal Ordnance, Israel Arms International, and Auto-Ordnance.

Example: Serial Number B311


Note the thin "reinforcement" of the front of the lug the trigger housing pins too.


Note the shape of the lug that engages the recoil plate.


Markings added to Receiver Ring, Changes on Receiver

Markings were added to the receiver ring by s/n B676. By C473 the thin "reinforcement" on the front of the lug the trigger housing pins too was expanded to the width of the lug and continued through end of production and onto the carbines made by Iver Johnson, Federal Ordnance, Israel Arms International, and onto Auto-Ordnance. C473 and C630 have a wide recoil plate lug.

Example: Serial Number C473


PLAINFIELD
MACHINE CO.
CAL. 30 M-1


Wide curved "reinforcement" of trigger housing lug


Wide recoil plate lug


Integral Recoil Plate, Similar Markings & Locations

The overwhelming majority of Plainfield Machine carbines utilized a recoil plate design based on the GI Carbines. Sporadically in the late C series and through the late D series some of the receivers have an integral recoil plate. So far this has not been observed in the the E series. This change in design works for certain other rifles but Plainfield no doubt learned fairly quickly the purpose of having a separate recoil plate was to absorb and redirect the shock generated during firing to prevent cracking the stock in and around the recoil plate screw.

Example: Serial Number D620


Notice the change of font and spacing of the words on the receiver ring typical of the earlier years at Plainfield.
The initials J.J.M. were a personal touch by machinist Jim McNultey, who ran the machine shop at Plainfield.

Example: Serial Number unknown


Notice the lengthwise cracks in the stock emanating from where the rear of the recoil plate should be.


Arced PMC letters, Serial Number in Two Locations

On Plainfield Machine M1 Carbines serial numbers G046 through G348 the PMC on top of the rear of the receiver was arced along the line of the bevel at the rear of the receiver. Some, if not all, have the serial number above the PMC letters in addition to the usual location on the left side of the receiver. The serial number on the left side of these receivers have a hole drilled and tapped into the letter G, obliterating part of the serial number. This scope mount hole drilled and tapped into the serial number sometimes appears on Plainfield carbines other than the G series, throughout production. Other than the G series, the serial number is normally not added elsewhere on the carbine. Other than the G series it is not known if Plainfield drilled the hole on the other carbines or if it was done by one of the carbine's previous owners.

By 1965 Plainfield Machine included scope mounts manufactured by S&K on several of their models but the S&K scope mounts did not require a hole to secure the front of the mount. The scope mount most likely used on these carbines was manufactured by Numrich Arms Corporation (Numrich Gun Parts) in West Hurley, NY.

Example: Serial Number G046


Yet another change of font and spacing of the words on the receiver ring typical of the earlier years at Plainfield.


Scope mount hole drilled and tapped obliterating the G


Note mark in dovetail consistent with a set screw to hold a mount in place.


Note position of mounting screws


PMC Letters Eliminated from Rear of Receiver

By serial number G990 the PMC letters were no longer placed on top of the rear of the receiver. The markings on the receiver ring remained the same.

Example: Serial Number H469


Standardized Receiver Ring Markings

Sometime between serial numbers K901 and L786 Plainfield changed the markings on the receiver ring to what would be used in that location throughout the rest of production. Slight variations in the depth, size, and placement are not uncommon. The 1 in M-1 sometimes has the line at the bottom of the number, sometimes not.

Section II

Serial Numbers 001 - End of Production

(1965-1978)

By the time Plainfield reached the end of the serial numbers with the letter prefix at Z999 in 1965 they had established a consistent method of receiver markings and location that continued throughout the rest of production until the company began the transition to ownership and operation under the name of Iver Johnson Arms.

The only marking changes that occurred on the Plainfield carbine receivers with a numerical sequence from 001 through end of production involved the serial number.

By this time Plainfield had begun to manufacture some of their own parts due to the unavailability of surplus GI parts. Over time PMC added, and sometimes ceased, marking their own parts with letters identifying the part as having been made by or for Plainfield Machine. Refer to the page on Parts & Markings for further details regarding parts other than the receiver.


Serial Numbers

The lowest numerical serial number data has been collected on so far has been s/n 109. The only markings on the entire receiver were the letters PMC on top of the receiver to the rear of the rear sight. The barrel was a surplus GI barrel manufactured by Inland during WWII, Given Plainfield Machine did not use surplus GI barrels and this is the only one like this with a numerical only serial number that has been observed so far, there is a good possibility this carbine was a one time occurrence and the reason to use caution with the words "never" and/or "always".

The next serial number and carbine data has been collected for is s/n 231A. Data collected after s/n 231A through s/n 1451A includes dozens of carbines. As mentioned on the page that covers serial numbers, use of this A suffix was inconsistent between s/n 231 and 1451 with at least one in this sequence having the letter P instead of the A. Many of the carbines with the A suffix are known to have been sold to law enforcement agencies but insufficient data exists to come to the conclusion this was the meaning of the letter. S/N 1451A was purchased retail in Italy. It is not known if Plainfield exported it to Italy or if it was sold to a law enforcement agency who later sold it to an exporter, wholesaler, or retailer.

Example: Serial Number 231A


box that s/n 231A was contained in when purchased new

Removal of the Oval Cut the Serial Number was Within

Serial numbers prior to 43000 (letter prefix series included) were stamped within an oval cut out. The oval appears infrequently between serial numbers 43000 & 63000 after which it was discontinued.

Example: Serial Number 157209


The Typical Plainfield .30 Caliber M1 Carbine: Serial Numbers 001 through end of Production

The markings, their location, the barrel, and the receiver were consistent from serial numbers 001 through the end of production. The quality of the finish and machining generally continued to improve over time.

Example: Serial Number 5136

This carbine was manufactured about 1966 and surplus GI parts include the slide, entire trigger housing group, rear sight,
and recoil plate. The buttplate is rubber with two screws. The recoil plate screw is a rounded Phillips head screw. Receiver
finish below the stock line is not of the same quality of the finish above the stock line. Later Plainfield carbines the finish
was the same both above and below the stock line.

Part IPart IIPart IIIPart IVPart V
History
of
Plainfield Machine Co.
1951-1978
Serial Numbers
&
Dates of Manufacture
Models
&
Flier/Manual
Downloads
Receivers
&
Markings
Parts
&
Markings