Those B-Square mounts were popular in the late 90's-2000's as a no-gunsmithing mount. As I recall back then they sold for $29.95-$39.95 retail, based on the finish. Yours being stainless would have been on the higher end of the price scale. Volquartsen, Clark, and MGW were other brands of Weaver no-gunsmithing mounts that were popular at that time since none of the Ruger .22 production target pistols came from the factory with pre-drilled scope mounts other than the Competition Models. Mine is currently sporting a Weaver Brand mount that attaches to the rear sight via the elevation sight screw hole. I bought the mount, just to try out the then new style mount, before recommending it to any customers. I was pleased enough with the mount's performance to keep it on this pistol.
Like you, I found my KMK one day at a gun show sitting on a table 'used' section with one of the area's larger regular retailers. It was packed full of crud, looked like it had never been cleaned by the former owner. I knew immediately what it was, but played it very low-key with the seller. I picked it up for $225, which was a ridiculously low price at the time - I couldn't get the money out of my wallet fast enough when I asked for his price on the old MK II, and didn't even try to dicker with him. I can't imagine ever parting with it, I like the feel of this little pistol so much. A plus, this one is dammed accurate for a production gun. I have shot it in many local Bullseye competitions. It hangs in there, accuarcy-wise, with some of my more high-dollar match target pistols. I'm sure you'll get many years of enjoyment out of your new/used KMK too.
R,
Bullseye
