I finally got around to snapping some pics of the examples I have from each of the M1 Garand manufacturers - International Harvester, Winchester, Springfield Armory, and Harrington & Richardson. I've included one below of the heel stamps:
The IHC is from early on in their production run (circa 1953); the WRA is a mid-war issue (May 1943); the SA is a late-war issue (Nov 1944); and the HRA is from later on in their production run (circa 1955). The WRA is nearly all correct, including the lock bar rear sight, and the IHC is also nearly all correct, while the HRA is mostly correct and the SA is an arsenal mixmaster. Each is sitting in a stock from the correct manufacturer, so you can compare the size of the back porches (the area of the stock under and behind the heel of the receiver). The HRA has the fattest back porch, and this helps when ID'ing an unknown USGI stock; the WRA has a fat back porch, but this is not a reliable indicator - you need to look at other clues in the stock's lines and characteristics; the IHC has a scalloped area on the right-hand side, and this is somewhat reliable, but the presence of an alphanumeric code stamped in the barrel channel is more definitive; and the SA has a smallish back porch, though again this is not a reliable indicator, especially if the stock has been through the arsenal process where sanding was done.
In case you were wondering, I edited out the second part of each S/N
The IHC is from early on in their production run (circa 1953); the WRA is a mid-war issue (May 1943); the SA is a late-war issue (Nov 1944); and the HRA is from later on in their production run (circa 1955). The WRA is nearly all correct, including the lock bar rear sight, and the IHC is also nearly all correct, while the HRA is mostly correct and the SA is an arsenal mixmaster. Each is sitting in a stock from the correct manufacturer, so you can compare the size of the back porches (the area of the stock under and behind the heel of the receiver). The HRA has the fattest back porch, and this helps when ID'ing an unknown USGI stock; the WRA has a fat back porch, but this is not a reliable indicator - you need to look at other clues in the stock's lines and characteristics; the IHC has a scalloped area on the right-hand side, and this is somewhat reliable, but the presence of an alphanumeric code stamped in the barrel channel is more definitive; and the SA has a smallish back porch, though again this is not a reliable indicator, especially if the stock has been through the arsenal process where sanding was done.
In case you were wondering, I edited out the second part of each S/N