mkcharlie, does it look like this?
Mine is a 6690, which as Forsmithers said was a transitional gun between the 2nd and 3rd Gen. guns. Basically a 3rd Gen frame with a 2nd Gen slide, barrel, and sights. The SCSW doesn't give any actual number as far as production, but the research I did on the 6690 indicates somewhere between 400 and 900 were produced. S&W is known for using up spare parts, which in this case, they did for these transitional guns. RSR was the sole distributor as demonstrated by the serial numbers, all starting with RSR. I haven't heard of the 6990, but there were a number of variants depending on what slide components and size of frame were used.
If the one you're looking at is anything like mine, they are great shooters and would be worth having.