That is similar to the fm/90 prototype sniper rifle in 7.62x51 NATO caliber with Swarovski 6x40 scope & threaded muzzle . Never adopted . SARCO had several of these for sale a few years ago . I do not remember the price , but I think maybe $6000 . Does anyone else remember the correct price ? SARCO had several sets of the mount & rings for sale as well . Got a set , but never made a project out of them . The base wraps around the receiver & is screwed in on top of the receiver . That tightens up the action because of the thumb cutout in the receiver , or at least that was the idea . The stock needs relief for the base . I will see if I can find photos ?