have SKS, SN1608852,from CJA,SFLD,MI, has plastic stock with plastic folding butt and bayonet. Have 2 mags that don't work... 30rd USA,metal, and 20rd, plastic Pro Mag for SKS-5A. Don't know what 5A indicates. The USA must be forced in position but I can fire with it. The ProMag does not fit.Bass Pro Shop suggested I get PAPCO mag with their sku of 1903011. Any suggestions?