The serial number goes to 1965 which agrees with the 15-2 model designation used from 1962-1967.
HOWEVER, a Model 15 is the Combat Masterpiece standard with 4" barrel.
A 6" barrel was standard on the Masterpiece Target, commonly known as the K-38.
Guns do get mismarked and barrels do get replaced, so that is only a minor oddity.
Resale value depends on condition, somewhere in the hundreds of bucks.