Most internal parts on the M1 are marked with what is known as a drawing and revision numbers. Each contractor (SA, Winchester, etc), used a specific range of drawing and revision numbers for each major part for a certain time range. When the part was changed, a new revision number was assigned to the drawing number. In researching the drawing number on each part, you can tell whether or not the parts are the correct vintage for your March '41 M1. It doesn't guarantee that the parts are original to the gun, just that they are the correct revision for that rifle.
Here are a couple of websites I used when researching my SA M1:
http://battlerifle0.tripod.com/index.html
http://myplace.frontier.com/~aleccorapinski/id14.html
Mine is pictured below. The receiver is dated Jan 1941, and when I got it all but possibly one component (I don't recall which, and I think I may have later corrected myself - it may have been the right vintage) were the correct vintage. Even the barrel is original, or at least the correct date for my rifle (Jan '41), and though it is dark and fairly heavily pitted, it still shoots quite well. The stock has very faint remnants of inspection cartouches (as if it were sanded and refinished at some point), and it could be the correct vintage (I really can't be certain though). It could even be the original stock, according to a local gunsmith who specializes in Garands, but there is no way to know.
Anyway, point is it is loads of fun to research.
I did have to replace the op rod catch (it fractured at the point of one of the pin through holes) and the trigger guard (the locking lugs on the guard were worn enough that the action would come unlocked from the stock during firing), but I made sure to replace it with the appropriate drawing number, and kept the original parts.
It is a very fun, addictive gun to shoot! If you choose to shoot yours, be sure you select ammo that is safe for M1s. Surplus M2 ball is a good choice, for example. Most commercial loads are a little hot and can bend the operating rod.