Yes some 54 and all 55 and up 261s I believe will / can have a neoprene seal. . .
"can" might be correct, but "will" is not assured:
0.137 Seal: graphite-impregnated 1940-1957 that Parts Manual was up to 1957.
I'll look that part number on the GM parts wiki and report on later 261s. I think I used a rubber seal on my 1960 261 that I rebuilt in 1972 (and, it is still running with no leak).
added later
GM/Chevrolet part #: 3836774 [correct code is 3835774]
I cannot find a "cross listed" modern part?
Jim Carter Truck Parts shows a variety of types/brands of 261 rear main seals
