Hugh,
As a kid watching these engines do switching duty at the Mott Haven yard, I never realized that there was such a thing as an S-3. The end platforms would have been an obvious difference, but I don't ever remember being observant enough to spot the difference.
I discovered that class engine as an adult in the 1980s when I bought a brass HO model, and it turned out to be an S-3 with the end platforms.
Most Mott Haven photos show S-2s working the yard. Staufer mentions that the S-3s were the only S class engines to retain steam heat boilers to the end so were used in passenger service.
But, I never saw an S running through the Mott Haven yard. They would shuttle trains to and from the yard to Grand Central but never on through trains in the late 50s - early 60's when I did most of my train watching.
There was also a class S-2A which was slightly lighter, but not sure of any spotting differences.
According to the roster in Al's book, there were 1 - S-1; 32 - S-2s; 2 - S-2As; and 12 S-3s.
I haven't seen Al since he stopped going to York. Back when I was doing a little painting in acrylics, I would always stop by (in the purple hall at that time) to admire the paintings that Al and his son brought for exhibit.
Jim