These are late war and post WW2 production type with the bottom plates held in place by four tack welds and large spring catch. The earlier WW2 followers had the problem of the plates coming off so four tack welds were needed instead of two (See photo two).
The last production seems to have been in January 1950 per original Remington box note by a counter with the nickname "Kansas City Kitty". (See photo three)
These are original new unissued parts.