I'm left wondering about the interrelationship of these late forties "S" series to the Short Action Model in "C" series.
The short action was implemented
during the S prefix series. Specifically, the first production example of the .38 M&P with the "high speed" hammer was serialized at S990184. That revolver was assembled in March, 1948, and shipped to Mesbla, Brazil on April 7, 1948. From that serial number on, to the end of the S prefix series, the vast majority of .38 M&P revolvers had the high speed hammer installed.
When the first C prefix M&P was assembled (March 22, 1948), the high speed hammer had become standard. For all intents and purposes, all C prefix revolvers had the new short throw hammer. I have located only a single C prefix .38 M&P with the long action and it appears to have been a special order that left the factory on April 20, 1949. This, of course, is not to discuss the rare .32 M&P, which is a bit of a different animal, although built on the same frame.
the earlier "C" Models originating in '49. Were some of these two models actually overlapping/competing?
Yes, assembly did overlap slightly. That appears to have only happened for a time in March and April, 1948. I've found no evidence that S prefix guns were assembled after April, 1948.
I don't know how they could be thought of as "competing." They were the same model gun. The only difference at that point was that the S series of numbers had hit the limit of six digits (S999999). So, the serial sequence was replaced with the C series. The guns did not actually change in any way at that point. As related above, the "high speed" hammer had already replaced the older long action. No new engineering changes of any significance were made again until 1956, when the fifth screw was eliminated.
'IF' so, presumably to emphasize frame size differences in marketing???
I can't imagine how you would come to this conclusion. The frame size is precisely the same. All of them are K frames.