Per 3rd edition of Standard Catalog of Smith & Wesson, Supica & Nahas: Model 15, The K-38 Combat Masterpiece
"Originally offered with a 4" pinned barrel, and then offered with a 2" pinned barrel...later offered in 1986 with a 6" or 8 3/8" barrel, all of which were machined with a narrow rib on the barrel"
So the primary difference between a Model 14 with a 6" barrel and a Model 15 with a 6" barrel was the width of the barrel rib.
The standard front sights were also different
The front sight of a Model 15 was the Baughman Quick Draw front sight on a plain ramp.
The front sight of a Model 14 was a Patridge front sight (1/10" or 1/8" wide)
S&W offered a variety of custom front sights, but these were the standards per the catalog
FYI: the Model 15 with 6" barrel (and 8 3/8") was introduced in 1986 (Model 15-5)
Engineering change to Model 15-6 in 1988 (same time that the 8 3/8" and 2" barrel were discontinued)
The Model 15 with 6" barrel was discontinued in 1992, a short production run compared to the Model 14