Large Caliber Headstamps

I tried this on B.O.C.N but not any answers…

Just starting collecting the bigger stuff. I have two German rounds that I am confused with.
At 6 o’clock on both is BV in a circle. Is this the manufacturer and who is it, or is this an ordinance code.
At the 12 o’clock position, one has a BM in a circle marking, the other has an M in a circle marking.I know one is the manufacturer (loader) the other a component provider but which is which and who are they.
Added: the cartridges are 15x96 MG

adding some pics

71 eagle inspector mark

have another that has a 71 eagle inspector mark

71 eagle inspector mark…This one is a 20mm Oerlikon cartridge.

71 eagle inspector mark

Can not determine inspectors mark