They're an interesting Colt to research. You should definitely get a letter for yours if you don't have one! The 1928 serial number sounds correct. As Bill mentioned above, these pre-Detectives were made in 1926-1927, however, since they were a special run of guns, Colt used a future serial number range, and so they all come back as saying 1928 even though they are earlier than that. I had Frank Glenn look mine over too, and it was in surprisingly good condition. Glad yours didn't need much either.I guess I have more research to do on this one! My serial number is in the 3343XX range..maybe a colt letter on this one. Thanks for the info.
Since we're showing off our pre-detectives, below is mine with its Colt letter. You'll notice it says Colt Detective Special, but it is actually a 2" Police Positive Special, and Paul since then sent me a new letter to fix the naming error. Yours should come back as a Police Positive Special, too.
Later today when I get the chance, I may create a new post in the revolvers section to continue this topic. Been meaning show mine off and see what new information people have to share, as it's been a while since the last time we had a good thread about these unique revolvers.