The 915 was S&W's first attempt to offer a more budget-priced semi-auto. They did this because they were starting to get their asses kicked by Glock. The frame had fewer machining cuts than a 5904, it lacked an ambi safety or fancy sights, and the finish was a basic matte blue (which rusts pretty easily, FYI). The model was only offered 1992-93 IIRC.
The 910 replaced the 915 in 1994 in the wake of the assault weapons ban which also banned +10 round magazines. Besides being shipped with 10-rounders (hence the name) S&W also made even more changes to simplify the design and keep it competitive with Glock. It was full of plastic and MIM parts, the slide and frame profile was very crude and boxy, and the barrel lockup was simplified with no more locking lug. Despite all these changes it was still a good shooter, if uglier.
I used to have both this 915 and 910, but later sold the 910 which I somewhat regret.