In 1938, The SS was not part of the German Army. It was a special corps with a dedicated budget. The same year, the manufacturing of 7.92 ammo was requested to train this special corps. The logo SS was stamped to track the ammo and according to the contract. 
Those bullets were only made in 1938 with the SS stamp afterward germany will enter war and SS divisions were integrated in the army.
As found condition, rare though.