That's Smith & Wesson slide and all S&W parts moved on to and in to an aftermarket (and likely stainless steel) frame. The most interesting part is that unless a true serial number is located somewhere not shown in the pictures, "PROTO 5" is the serial number of record and would seem to be an early (wait for it... prototype
) frame from that company.
I've seen the frames and pistols with those frames before, I believe it was a California manufacturer. I have not yet seen any increased or collector premium value, but it is surely interesting.
As to that value, I think it makes sense to add that the S&W Model 59 has never been a highly valuable collector kind of piece, which translates to good news -- because if it were some extremely rare, highly valuable and sought after model... an aftermarket frame would destroy that value. But with a 59, it simply make it a cool curio and a heavier duty pistol than you can get in a 59.
There was a stainless "59", extremely limited and actually named the Model 147-A. Only the frame was different, so there is basically zero chance that your pistol was anything other than a normal 59.
Somewhere in the world or in history, there is or was a Model 59 that is missing every single part, because it is now in your "Proto-5."
Neat!