The original box had a printed end label that would say Product Code 104200 on it if it was shipped as a Mountain Gun. Does your box still have the end label on it and does the gun's serial number match the serial number on the label?

Your gun is unusual for a 686 Mountain Gun. I don't think they were made before the 686-5 variant came out. Yours is clearly earlier than that.

I wonder if your gun had the original full-lug barrel replaced with the tapered Mountain Gun barrel it has on it now. This was done by 686 fans as S&W had some extra Mountain Gun barrels in stock in their service department for a while.

The newest reference book for recent (post 1997) S&W is the Standard Catalog of S&W. The newest edition just came out and doesn't say how many were made, just that it was a limited edition made for the Stocking Dealer program S&W had at the time. Previous edition doesn't say how many, either.