WWII vintage rifles in their original state should be found with a milled trigger guard and later models may have the stamped type. The best way to distinguish the two is that the milled type has that hole in the guard being the trigger where it clips into the receiver. The stamped type doesn't have this. The safety on the Garand has a matching hole through it so basically, you see a round hole in the front of the trigger and the back for a milled, and only a hole in the front if it's stamped. Pretty easy to spot.
The document seems to be showing the results of testing some different stamped trigger guards in 1942, anticipating the need for faster production now that we were engaged in war. Apparently, there were some problems with the thinner and more flexible stamped parts coming loose! The way the trigger guard clips to the receiver, it has to be flexible but rigid enough to not break loose when the gun recoils during fire. With the stiff milled trigger guard, that's no problem. I prefer the milled trigger guard and I'm glad they kept it that way for quite some time. Even some post-war rifles had the milled trigger guards so I take it that the change wasn't considered important enough during the war. In my opinion it wasn't needed at all. Even my later Korean War era rifle has a milled trigger guard and CMP's site shows the hole I mentioned in use for mounting a winter trigger mechanism.
http://www.civilianmarksmanship.com/...intertrig.html
I don't believe that's what Garand intended the hole to be used for but it's an interesting idea.