Mine is a Model 15-4,,,
That means Model 15, 4th revision of the design.
Unfortunately I am not near my S&W book,,,
I can't tell you what dates the revisions were made.
S&W was kind of funny about barrel lengths,,,
For instance the Model 15 was catalogued as a 4" barrel,,,
But you do see 2" barrel models showing up at auction all the time.
I saw a 6" barrel version at a gun show one time,,,
But I think it was re-barreled with one meant for a Model 14.
Basically none of the model numbers had a specific barrel length,,,
A most common barrel length for sure,,,
But a single length per model,,,
Nope.
Now watch,,,
Having said that,,,
One of the real experts will prove me wrong.