Complex sentence: The officers of the SS were almost all of lower middle-class or working-class origins, who would never been considered for commissioning on the old German Army; but in the SS they could take part in the "gentlemanly rituals" of the mess.
Split into simpler sentences: The officers of the SS were almost all of lower middle-class or working-class origin, who would never been considered for commissioning in the old German Army. In the SS they could take part in the "gentlemanly rituals" of the mess.