There is one exception to the potentially interchangeable rule that I know about - the 80mm 'S' has a different viewing lens construction from the taking lens. The 80mm f3.7 uses a different (Copal) shutter and may also be non-interchangeable. This is not to say that the viewing lens is always as good for photographic purposes, but anecdotal evidence suggests that both optics are of similar quality.

Graham