Hi Anders ,i think the answer to your question is in the M1 designation on the box ,ive included a few pics of some boxes in my collection .The 50 round Winchester box is stamped 8 MAY 1941 ,LOT 28 and the 64 round box ,1942 ,LOT 87 .The 50 round box for .45 s is dated LOT51 ,NOVEMBER1940 ,D9RM41.
Western Cartridge Company bought out Winchester in the mid 1930s and had plants at East Alton ,Illinois ,New Haven ,Connecticut and St Louis .
I dont know when the M1 designation came in ,but iam sure someone will on site ,regards Dave