Yes, the "S" in the model name of 908S stands for a stainless steel slide (and a "light" anodized aluminum frame finish).
The 908S was offered for those customers who wanted something other than a carbon steel/black slide finish.
They did the same thing with the compact .45 model 457. They offered the 457, which came with a carbon steel slide (and black finish frame), and then they offered the 457S, with a stainless steel slide (and light finish frame).
The Chiefs Special line was just a little different, as each model had a letter identifier for the slide steel. For example a CS9C meant a carbon steel slide (and black finish on the frame), while a CS9S meant a stainless slide (and light frame finish). Ditto the CS45C or CS45S, etc.
The 908's were pretty durable compact aluminum-framed 9's. The didn't offer all of the "pretty" exterior machining of the 3913/14 models (nicely radiused corners, etc), and they made some changes to save some machining/manufacturing steps (such as the plastic sights and recoil guide rod, lack of a barrel lug, non-adjustable mag catch button, to think of a few), but the guns were reliable and durable "working" pistols.