I saw this rifle recently, it was marked 'Reising .351' on the barrel, and '.351' on the bolt and the cartridges were headstamped with .351 SLR ( Ammo looked just like pictures I've seen of .351 Winchester used in their model 1907 self loading rifle )
The rifle had what appeared to be about a 7-10 round magazine which had two vertical grooves on each side as did the mag well. The location of the mag well and the mag release look very similar to pictures of the Model 50, 55, and 60 Reisings that I've seen, as well as the H&R Leatherneck .22 rifle. The charging handle was accessed through the bottom of the stock forend. On the rear of the receiver was a very nice Redfield peep sight.
So far I have not been able to find any information about this piece and am dying to know more about it, thanks!