i would bet it's an old arms or ggg mount. but i have heard of some custom mounts being made in the days before these mounts were available. so anything is possible.
FWIW: i looked at and held a LRB integral mount reciever at camp perry. this was a pre-production sample and had not been heat treated. thus the color was jet black. unlike the standard receivers which are more of a gray-black. it was interesting to chat with lou about how the hardness causes a slight color change. as he had several standard receivers and there were slight color changes in those.