Whether the adjustable sighted gun is a Dayton Model is beyond my competence, however the OR CO CA is almost certainly Orange County California. I recovered a duty gun stolen from an Orange County Marshal about 35 years ago. It was a 15, but while I don't recall the barrel profile or exactly how it was marked, the stamping on this gun is similar. In the course of tracking down the owning agency to return it (IIRC the Sheriff's Office had entered it into NCIC) I got the impression that both the Sheriff's and Marshal's issued 15s. What I recall more than the barrel profile was the superb action.
I agree, $600 is pretty rich even if left as is. There are better 15s out there for better prices. I don't think that's a Model 10 heavy barrel due to the rib. It also doesn't appear to be a cut down Model 14 barrel, since the markings look centered. More pictures would be helpful, but if you are willing to spend $800 on a 15, keep looking.