There is nothing in Mischinger’s fine books about any round like this in 7.9 x 57. I have about 40 DDR 7.9 x 57x in my own collection, and have never seen or heard of a grenade blank or any round of this description.
There is an American company that makes blanks and uses a white tip seal on lots of them. Perhaps they or someone else bought up a lot of the ball ammunition in this caliber - it was around in quantity very cheap, as you know - and turned them into blanks. The loaded ball ammo was cheaper per round, considerably as I recall, than new commercial brass in that caliber.
If so, this would be a movie or reenactor’s blank, almost certainly.
Just my thoughts on the matter. I have no documentation for the use of DDR cases by anyone to make movie blanks.