David, this lens brings back fond memories from when I was the Marketing Manager for Interphoto Corporation in the '60's.
We sold this lens under our Accura name, while our wholly-owned subsidiary, Spiratone, sometimes sold it under the same name or under their own private label.
The lens was designed by Michihiro Yamaki, the founder of Sigma in 1961.
He designed several different fish-eye lenses during the fish-eye craze in the 1960-1970 SLR era.
I first saw a sample of this lens around 1965 and tested it on a Nikon F with Kodachrome.
As I recall, we sold it for about $100 (retail), with a T-Mount attached, while Spiratone sold it without an attached T-Mount.
I'm glad that you have that impossible to replace front lens cap!
Good luck with the sale.