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. Serial numbers ran consecutive and changes in markings or receivers were not identified by any particular serial number sequence. Not until serial number H001 did Plainfield remain relatively consistent with their markings and the receivers they were on. Many of the photographs depicted below were submitted by owners who offered to share them with you. The photographs were selected based on what they show as opposed to quality. Different people have different skill levels and I am indebted to everyone who has shared, regardless of their skill level.

Plainfield's First .30 Caliber M1 Carbines

The highest serial number observed on an H&S carbine so far has been A615. The lowest serial number observed on a PMC .30 caliber carbine so far has been A711. About four years later Plainfield produced carbines in caliber 5.7 Johnson using an A prefix serial number. The 5.7 Johnson caliber carbines were consistent with the M1 Carbines produced by Plainfield during that time and significantly different than the first carbines made under the Plainfield Machine Company name.

The markings initially used by Plainfield Machine Company (PMC) were a continuation of those used on 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.

Markings added to Receiver Ring, Changes on Receiver

Markings were added to the receiver ring by s/n B686. 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.

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 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.

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.

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.

Standardized Receiver Ring Markings

Sometime between serial numbers K962 and L220 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.

By the time Plainfield reached the end of the serial numbers with the letter prefix 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 0008. Sporadically during the first 1500 or so carbines absent the letter prefix there appears a single letter suffix. A being most common letter after the serial number with the letter P observed as well. 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. It's also possible the initial intent was to continue the use of a letter as part of the serial number starting with the letter A.

Removal of the Oval Cut the Serial Number was Within

Since their first carbines serial numbers were stamped within an oval cut out. The presence of the oval cutout became intermittent about s/n 39000 becoming rare after s/n 47000.

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.

Part I	Part II	Part III	Part IV	Part V
History of Plainfield Machine Co. 1951-1978	Serial Numbers & Dates of Manufacture	Models & Flier/Manual Downloads	Receivers & Markings	Parts & Markings