I was a bit surprised by such a short sentence and he's probably going to be out before he serves 6 months of it, it does send a message that they are anything but tough on this kind of crime.
I think (but cannot confirm) that part of this is what he plead guilty to and the sentencing at the time of the offences (being some 30-40 years ago) is less than guidelines today.
I appreciate that sentencing may have been different back then, but they could have given him non concurrent sentences of varying amounts and still added up to a lot more than 15 months, which is after all little more than a month for each offence, and much less if they release him early.
Those that we know of, of course. I would suspect there are plenty other victims.