“I don’t think the ‘tube’ in the stock is an actual magazine, but perhaps just a place to keep a few spare rounds... The actual magazine is the tube under the barrel as with modern carbines. If you look at that x-ray upthread, the bored hole in the stock stops well short of any mechanism...”
I agree. Also, the butt plate looks more substantial than any thing I can find in a picture or exploded diagram on the web. Possibly it allows for some hinged opening that can’t be seen in the x-ray.