There was a thread created and some numbers collected (and others that have cropped up)
and I is made somewhere not by Rocky Mountain (probably, maybe, who knows)
91 being 1991
09 being month
the rest maybe just a number in the line.
All guess work really and it's just what seems to have turned up, earlier 'month' numbers where definitely that year model.
Late one would be the next years' model
Of course nothing conclusive and I think the Equipe 1993 that cropped up actually had a I9100 code or something. Hence not conclusive.
I9110 and I9201 where 1992 sort of thing (model years tend to run similar to, at least our, academic years which start ~September.)