Random Guy makes some really good points and honestly, each era of the M40A1 build program was a little different. It just depends on which era you want to build it to.
The "official" M40A1 build guide is from the 1990's, when the shop was still the RTE shop and they were working on the C prefix or Charlie receivers. In that book it details they are supposed to inscribe the US mark on the receiver (as seen by Random Guy), and then mark the last four of the serial on the Bolt, trigger guard, floorplate, stock, and the mount. Then mark the barrel with the RTE proof mark. The original builds in 1977 had variations to the above.
The E, or Echo receivers came in after the shop became the PWS shop circa 1999ish. There is not an "official" manual I have seen for the PWS changeover and E receivers, but I have seen a lot of the E "Echo" M40A1 builds. I've also seen a lot of small notes here and there on the changes in the builds in the Marine docs for these receivers. There were a lot of variations in the Echo Receivers with the markings. Some barrels were dated, but then it was determined to be redundant because the rifle book said the same info. Some Unertl mounts were marked with the serial, most weren't. Some barrels were hand-stamped with single stamps to write out the PWS-P as the proof mark, some had the later PWS stamp after they had one single stamp made that said PWS.
I think the reason there was so much variation in the Echo receivers was because the Marines were already done with the M40a1 by that point. They were already focused on the M40A3 and working out the flaws in that rifle system and the M40A1's were only being made because they needed rifles and the M40A3 wasn't fully operational yet. So the Echo M40A1's were built after they already knew the M40A1 was basically done and they were switching to a new design. The Echo receivers were just a temporary fix to a dying rifle platform. I think that is why there is so much inconsistency in them. You also see a lot of recycled parts in these.
On a side note, looking at all the M40A1 serial numbers, and M40A3 serial numbers, I don't really see any M40A1's that were converted to M40A3. But that is also detailed in the work orders and Marine docs. I know Ken Davis had told me initially they wanted to keep the M40A1 and M40A3 programs separate and start with all new receivers on the M40A3 program and that is the same thing I see in the Marine Docs as well. The work order to build all the new M40A3's, was followed up with a contract with Remington to buy all new receivers.
One final thought. By the end the M40A1 program, it became a program of cannibalization. They would break down rifles for parts to keep other rifles running. So that can create a lot of variation for all the different eras of the M40A1 build program.
So when I was examining all the M40A1's in Marine inventory, you would often see some random part that seemed to not match up to the era. Even though there were guidelines to what was correct on these builds, Marines are Marines and parts are parts. We as collectors like to clone them perfectly to what our knowledge of them is, but the Marines were only worried if they could shoot accurately and if they functioned or not. I did see a lot of recycled parts in the builds.
