That is a stainless aftermarket frame for a model 39 sold under the name "B.I.I." I don't think BII was related to S&W. I owned a gun built on that frame. My gunsmith at the time, used the BII frame, and all the rest of the parts from a regular blue Model 39 to create a functioning firearm. Mine had a two digit serial number, and I have only seen one or two guns built on those frames since. Back in the day, rumor was S&W had done some legal stuff to prevent continued production, but I don't really know if that's what caused them to disappear so quickly. Someone posted photos on this, or some other forum, of one that was highly engraved. These frames were produced long before there was such a thing as an all steel 539 or 639, approx. 30-35 years sgo. Back then, steel framed S&W 9MMs were not commonly available. I'd heard of factory steel framed 39s back then, but never actually seen one. The only thing really noticeably different on the BII frames, other than the stainless color and weight, was the 1911 style trigger guard. The finished frame appeared to be of high quality, precisely machined, etc. Mine worked very well and I wish I'd hung on to it. Anyway, neat paperweight, not everyone on your block is gonna have one of those...