Without doing a lot of research I don't know but here is a link the Rimmers Spitfire gearbox pages...
http://www.rimmerbros.co.uk/ItemList...3-Rail--m-3167
Ben Caswell is your man, he would know off the top of his head, I am sure.
I have what I believe is a freshly factory reconditioned Dolomite 1800 single rail with a J Type o/d and the two letters nailed the exact box. I don't want to part with the Dolomite box right now because I might need it, it fit's my 1275 'A' series BMC engine. I think the two letters are stamped on the top next to the lid, from memory but your box may be different.