the 15-88, 89, 90 are all the same machine, the last two digits denote the power source
88 - Treadle
89 - Hand Crank
90 - External motor
I converted an 88 to a 90 and gave it to a 12 year old budding sewest. Along with a full set of attachments, and she loves the machine. I've still got the treadle base out in the shed - the wood was ruined.
The 91 is the same basic machine, but has the potted motor on it.
According to ismacs, Singer made this style from 33-41, and the potted motor version from 30-56.
Most of the ones before that are the 15-30 group, made from 1895 to 1933, though there are a few other variations of Domestic machines in there.