As it went for auction, they provided some high res pictures.
Link to auction: https://www.screenused.com/index.cfm?&item_id=3025 ($70 000 asking price!)
The Rexim-Favor was a Spanish-made, Swiss-marketed, and allegedly French-designed submachine gun produced during the 1950s. Only about 5000 were made in total, as the gun failed to procure any significant military or police contracts. Mechanically, it was a pretty typical submachine gun, using a simple blowback mechanism chambered for 9mm Parabellum ammunition. It had a quick-detachable buttstock, and of some interest an easily removeable barrel as well. The barrel was secured by a large external nut which allowed quick removal for transport or changing of barrels. It also fired from a closed bolt, which is a bit atypical for guns of this type. That allowed better practical accuracy, but was also much more expensive to manufacture.