Don't know if you trust Law on this one, but 3 of the 8 he shows for '41 are flat buttplates.
He also shows a '42 Ar with the flat buttplate. I'd guess they just used up what they had.
Never thought much about it. I'll try to pay attention and see if I can find examples.
Found this from Gunbroker, '41 with flat. He says matching stock, so it would be my fast example #1.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=77894941
Quick run thru my pictures shows a '41 byf with flat plate and its stock matches. Have pics of two '42 Ar with flat stocks, although my notes say 1 is mis-match and there is no pic of stock serial. So can't really count that one, but the other '42 Ar my notes show as matching and photos appear to be matching although the owner blanked out the serial.
I'm done on this one. I think there is probably more proof showing '41 and '42 can have flat.