bullfrog,
What you are seeing is just the difference between the milled trigger guard and the stamped trigger guard. Both are USGI and are interchangeable.
Springfield Armory (the REAL one) and Winchester both made the milled tg's up through WWII. Springfield started making the stamped guard in mid 1944. ALL Winchester guards are milled.
As for H&R and IHC, they did not make Garands until the 50's. All of their tg's are stamped.
Re the cutout in the stock at the back of the guard.... that is there to allow mounting of a milled guard. All stocks have it, even the ones made later on for the stamped guards. This was done intentionally so either guard could be used during armory repairs & refurbishing.
I have one IHC Garand that was rebuilt at Letterkenny Depot in 1965. New 1965 SA barrel, re-parkerized, patces made on the stock, and has a Winchester milled trigger guard.
During the "Clean and Repair" program instituted by the Army, rifles were brought in from the units and evaluated. If there were any bad (out of spec) parts they were thrown out and replaced with the first good one that came to hand from the repl bins. Some parts may have been new and unused, others came from rifles that were scrapped because of a bad receiver.
A LOT of Garands went through this program. Some came out with only minor parts replacement, others, like my IHC, got a complete rebuild.
In case you had not noticed, all of the M1-A's also have the late Garand style stamped trigger guards. The M14 never had the milled type.
my .02 cents
Swampy