75b SA appears to have the 'same slide design' cosmetically as the standard 75b...
Same only in the full-size .40 versions, I think... I forgot about that when I replied.
The 75B DA/SA shown in the photo above looks to be a .40 -- as it appears to have the full-length guide rod at the front. That's not available with the 9mm versions -- hence a different slide. It would appear that the .40 DA/SA and SA models use the same slide, which explains the rails. (I've only had one 75B DA/DA in .40, and then only for a short while -- I wasn't into '.40' back then -- and I never paid attention to that part of the slide.)
The 9mm SA and DA/SA models don't (or didn't) share the same slide.
The extended slide rails are very slightly narrower ahead of the frame dust cover, so they may actually be there for looks, not function.
The SA beaver tail is a real improvement, and that's also seen in some of the newer models, including the new stainless models. (The beaver tail was the minor difference in the frame I mentioned, above -- I should have been more specific.) Newer models, like the Stainless models, also have reversible mag releases, not possible with the older 75Bs.