Excellent site. Unfortunately, unless I've missed it, the 80/2.8 C lens they describe is the old C lens with self-timer and default-interlocked aperture and shutter speeds. Those are pre-1984 lenses, as the CF's arrived in 1984. The 501C's "C" is 1994.
The 501C's 80/2.8 "C" lens is, at least outwardly, an almost-CF lens.
They couldn't call it a CF, not so much because of the missing interlock button, but because there was no F setting. The F setting on a CF lens allows that lens to be used on an F-body camera, like a 2000, 203, or 205. Actually, however, you can simply use B just as well.