These markings are not found just on 7.9 x 57mm ammo of DDR manufacture. German law requires that each and every round of surplus ammunition sold must show the caliber of that ammunition. If the caliber is on the headstamp already, no further action is required, that I know of. However, most military ammo doesn’t include the claiber in the headstamp, so this is the preferred method of complying with the law. The little symbol after the caliber marking, I think has to do with who marked them. I have not found any guide to these symbols. The one shown is commonly encountered, but is not the only one encountered. For me, the are a variation above those of the same headstamp without the marking, since they are done for an “official” reason.