Until the Quantum series, Infinity's product line was somewhat haphazard. This series appears to be contemporaneous with the original
Monitor series (Infinity Classics mistakenly bundles it with the SM series, with which it bears no relation), with its (mostly) Walsh tweeters. My guess is that
these would have been considered their entry level line (the 5000 being the top), with the Monitors being the higher end models. The
1500e is a mystery; I think that's a newer model (however, I doubt the date of 1988 given on that page is correct either).