There is only 2 options for the selector rail, suffix A, B & C and D onwards. The differences on a D on are; the front groove is wider allowing more lengthwise adjustment movement of the fork on the rail, the other is on the 3 grooves on top(1-N-2), the distance between 1/2 and N is larger on the D suffix as they need a larger depth of engagement.
If the early rail has been fitted with D gears it will jump out of 1st & 2nd.
If you look at page 37-20 there is some info about the setting of the 1st/2nd fork on the rail for D on boxes.
Hope this helps.