After closer examination, I think that I can answer my own question.

It appears that the barrel to receiver is a slip fit, and the barrel is held in and kept from turning by the barrel pin.

It looks like I will have to have a "gunsmith" shim the barrel to receiver joint, or to deform part of the receiver to prevent the barrel movement.

This looks like a very bad (cheap) design obviously.

Other than this problem, it seems like a very nice rifle.

I've got another one that is similar to it with a round barrel. It has the brand "Ranger" stamped on the receiver. I think this might have been sold by Sears or Montgomery Ward. This one is a little strange because it has nice bluing on all parts except for the receiver. The receiver is what appears to be "bare metal".