fat tom,
Well you certainly have come up with one of those extraordinary S&W anomalies that make S&W collecting so fascinating!
Sorry to be late to the party, but I think all of the possibilities have been mentioned already and I don't have any new or different theories. But I can offer my analysis and conclusion based on the following observations, and in agreement with David Wilson, Masterpiece and a few others, I'd bet a paycheck that you have a factory original Regulation Police with a 2" Terrier length barrel.
1. the matching barrel serial indicates to me it was factory assembled.
2. and the lack of a rework date on the grip frame and any other rework stamps like S, diamond, etc., in a period (the 1930s) when these stampings were as consistent as any other period, indicate the anomalies occurred during the guns initial assembly.
3. the 1936 era 2" barrel and 1930 era stocks, with flat silver medallions and what appears to be a gap at the front toe of the grip frame (because your earlier grip frame toe was machined round) indicate assembly took place sometime in the late 1930s.
Factory change order dated May 8, 1920; rebated Reg. Police grip frame toe left sq for easier stock fitting:
4. the 'out-of-normal-s/n-range' frame assembled ~16 years later has enough documented precedent, IMO, to be a realistic possibility.
5. in the pre war period a customer special order or employee owned RP in the configuration we see, would not be that uncommon.
6. likely not one of the 500 Post Ofc Dept. guns w/o the special marking or special flat steel plate on the butt for added weight, but perhaps an 'over-run' gun from that model, that are so often associated with a special production run model to insure the total order quantity will be met.
7. I too, am in agreement as others have posted, that a factory letter request is imperative for this gun as well as a subsequent SWHF request for additional factory documents.
In conclusion and barring further information to the contrary, I believe you have a righteous factory anomolie!
I hope this is helpful Tom,