I don't know about "slanted boxes", but there are three German 7.92s (actually more, if you want to count the 7.92x57J, but let's skip it for now).
You got your 7.92x33, which was made for the MP44. Little short thingy. Kinda like 7.62x39.
You got yor 7.92x57, which was the standard military rifle and machine gun round. That's the one you want.
You also got the 7.92 x 56. This is for the 1895 Steyr. It was a "substitute standard" for the 3rd Reich. So there is a lot of Nazi-marked ammo for it around.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.