The black primer seal by intself indicates a PmK. To identify a B-Patrone, which is another form of incendiary round and was at least initially classified by the Germans as a form of Ubungspatrone (practice cartridge) although it was used tactically, the black primer seal must be accompanied by the tip of the projectile being plain gilding metal and the lower 3/4 (approximately) of the projectile being chemically blackened, depending on when made. Early Beobachtungspatronen had a chrome tip along with a black primer seal. In both the early and late B-Patrone, the tip color or lack of color was about 10mm long by regulation, but in practice, these vary quite a bit. Of course, we are talking only of the German versions here.