Thanks to all for the comments.
Gil, good to know the likely source of the governor itself. The engine came from England, but if your friend was offering them for that long, then no doubt some would have ended up there (maybe even more than anywhere else). It is certainly a fine piece of craftsmanship. I have only seen a few S50 models with governors, apparently Stuart does not make them specifically for the S50 today, but maybe they did in years past.
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Hi Greg, Stuart never provided a governor for the S-50. It is 100% John Coleman's work. John was a very good model engineer
Gil