SAE 51 Prop Shaft?

Evenin’ all.
Does anyone know the difference between a 50 and a 51 prop shaft please?
I haven’t come across a 51 before, it is on a Cyclone 9 engine, but I haven’t been given any measurements for it. Apparently the propeller fitted would have been a Ham Standard 43D51, which again, I haven’t come across before.

Cheers

Pete