Fusil Asalto CB-52 prototype
Designed by Joaquín De La Calzada-Bayo at La Coruna weapon factory in Spain c.1943.
7,92x51mm M.52 30-rounds removable box magazine, gas-operated select fire.
I assume these things existed only as blueprints for almost a decade before being brought back to life in the work leading to the adoption of the CETME assault rifle.
Sauce : Forgotten Weapons, with the Spanish Ministry of Defense.