Upper is a S&W Model 909. Lower is a S&W Model 910. They sure look similar, don't they?
Could you tell I switched which one was on top?
From the back it's really obvious what the difference is:
The 909 is single stack and the 910 is double. Which didn't make a bit of difference in capacity in the mid-nineties. The "ten round" 910 magazines only hold nine rounds, just like the 909's! What you can do with the 910 is use a pre-Clinton 15 rounder.
Also of note, magazines for the 909 work in a Model 39, but not vice-versa.