Someone got a bargain...
I always lusted after a series 1. But at the time(early 1982) I bought the f/4 aperture with it's matched multiplier, because I wanted the extra pull that it gave me for wildlife photography. The lens was OK, even at f/4 but the converter was poor despite Vivitar's claims. Result was 2 stops of light gone and lots of blur. Was better using fine grain film and no converter. I decided after a shortish time that it had been an error, but too late to change without losing a lot of money. Ended up hiring mirror lenses, which weren't brilliant, but gave me the reach I wanted and eventually bought a Tamron SP 500mm f/8 mirror, which was fantastic compared to the hired ones - and most other lenses. The Tamron Sps were as good as the Vivitar Series ones, but with the advantage that the mount was interchangable. So you could put the mount on the camera and just change the lenses, although it was easier on Canon to remove lens plus mount. There were different versions of the adaptall mount, but the later ones were great.
The Makinon lenses were pretty good as well.
I ended up buying a canon 20-200 (roughly) f/4 about 10 years ago. Probably every bit as sharp as the Series 1, but not as fast. A lot easier to find as well....
Good luck on your lens hunt. Don't get tricked into paying too much by collectors.
Man is limited by his fears, not by his imagination.