Yes agreed John,DI did indeed manufacture WW2 7.92mm ammunition.

As mentioned earlier the IAA website has discussed this subject in length and is probably the best place to seek info without copying and pasteing all their comments here.

In summary it appears that the WW2 - MM 7.92 4X - DI headstamps were duplicated in the 1950's for 7.92mm production for covert use.....plus headstamp dates before DI was even making 7.92mm ammunition ?
This example with a 1940 date -


(pic from IAA website)

I'm sure the ammo gurus at IAA would be happy to debate any further questions relating to covert ammo

ATB Kevin