As far as I know, and this is from the deep recesses of my memory, the difference is that the RR sump has the main pan central with shallow sections front and back, whereas the SD1 has the main pan to the rear with no shallow section.
My MGBV8 had the sump with the shallow rear section, which allowed the exhaust to crossover into a single pipe on the nearside.
Scroll well down here for a couple of pictures:
http://www.britishv8.org/MG/DaveWellings.htm
