What other markings on the one with the disk? "Mecanico" is a distinctly Spanish word (slightly different spelling in Portugese), which narrows down the potential list of users. In the late 30's, the Soviets emptied their warehouses of random oddball captured rifles and handed them to the Spanish Republic to fight Franco. It was a way of transferring arms in a deniable sort of way.
If it is such a beast (and I'm sure someone here will correct me if I'm wrong), that rifle lead a life of legend. From England (surplus after WWI), to the White Russians (donated surplus to fight the Bolsheviks), captured by the Bolsheviks, donated to the Spanish Republican Army, captured by Franco's Nationalists, then dumped on the surplus market after WWII.