Any model you can't find on google (I am assuming you googled it), is very likely a private label/store branded bike. Store branded bikes from the 1970s were usually pretty crappy, but some of the store branded stuff from the 1980s and newer can be pretty good. It depends on the components, what the frame is made out of, and so on.

There's a lot of really nice 15 speed bikes out there and a lot not so nice. Hopefully you got one of the nice ones.

Well, a quick search on google showed a pretty good discussion on Sentinel bikes on this forum.