Mid 1940 Springfield M1 rifles

I have photos of two 1940 rifles, serial number 52112 with a S-A 7-40 dated barrel and serial number 52808 with S-A 8-40 barrel.

While the serial numbers are close to each other, there is a difference in the internal guide ribs. Serial number 52112 has an extra
narrow rib on the left side while 52808 is without the extra rib on the right side. These 1940 receivers show up with the extra rib
up to 65K while most receivers are without this extra rib. These are revision 2 receivers manufactured with the high guide ribs to
resolve the seventh round problem. The reason some have the extra narrow rib is still unknown ?

Any members here have receivers from this mid 1940 period with or with the extra narrow rib?Attachment 54036 Attachment 54037 Attachment 54038