Steven,
Just because a production change took effect in 1959, it doesn't mean that all guns in inventory at the factory at the time of the change had already been shipped. The 60-61 time frame is probably accurate. Only a factory letter will give you the exact date.
Mike
The 15-1 had the left hand threads on the extractor rod. If the gun was assembled with right hand threads, it would not have the -1. As the switch-over was made to the new version, old parts were used up; there was almost never an immediate change, so some Model 15 revolvers would show up in the same serial range as the early Model 15-1. There really is no mystery here.