According to Speed, Mauser introduced the slim style stock with grip cap at the very end of 1939. I have one that was proofed prior to April 1, 1940. I also have a standard style MM410B and the two stocks are vastly different. The slim stock has checkering on both grip and fore end, It has a plastic or horn grip cap, and the stocks on the slim rifles were not stamped with a Mauser cartouche on the right side of the butt stock. Due to the timing, it is doubtful that Mauser made very many of these rifles.

John