First, it is not a GI carbine receiver and your stock is not GI.
In the early 1960's a company called Rowan, Becker Co., of Waterville, OH, made carbine parts and also assembled carbines. At first they used unmarked receivers from National Ordnance, which they marked with their name; later they had receivers cast by other companies. One marking they used was "R.B. CO. INC." which might be what is on your carbine. I don't turn up any "RG" marking.
BTW, your front sight is on backwards.