Good luck with that-I never found anywhere to date my 310b. The best guess I could make was based on color, which was two-tone mint green/pale green. Riccar does still exist as a company (
http://riccar.com/) but all they make now is vacuums, I think. It still might be worth a call to them, see if they've got any info archived. I can't remember whether I did for mine or not.