Graham Patterson's Mamiya TLR page Dead Link Removed which is pretty much the authority on these things, suggests that they may not be interchangeable. For the 80mm S lens it says " 'S' version does not have markings surrounding the lens front elements. Lens details and serial on top of viewing lens. The viewing lens is of different design from the taking lens, and elements are definitely not interchangeable with the taking lens." For the standard 80mm lens it just says updated to become the 'S' version.
I don't know if the fact that the warning about not being interchangeable is only on the 'S' version means that it is interchangeable on the earlier version, but it could also mean that neither is interchangeable. Certainly all other characteristics are the same. Check which one you have and go from there.
As it is unlikely you would be able to sell a lens in that state, you could try giving it a go anyway and if not, just get a new one. The 80mm 2.8 is the most common and probably cheapest lens out there.