Looks like a 740 rebuilt on a 760 receiver. As long as it works, I'd say you paid a fair price. My LGS has a couple 740s and a 760 in thr $350 - $400 price range.
If you take it apart, you should see the indents inside the receiver where the slide rails would travel. As long as the bolt doesn't slip down into those, you shouldn't ever have a problem, and I can't think of any reason the bolt would slip.