I posted this on one of the Facebook groups with no luck, but some good suggestions.
I have seen several other examples of these same casings over in Europe, mostly the UK. Some variants had filling plugs in the base along with the primer.
It measures 95x305r, and is made from a cut and necked down 1969 British 105mm tank casing.
The British factory number, RLB lettering and lot number have all been struck out, and a German “DM40” stamped under the British factory number.
The primer well has been completely drilled out and no longer has the American 60% threading and is now smooth.
Most interestingly is the very large device stuck inside the casing. It looks like it would have been held on by the primer but is now loose. It’s made of 6 long U shaped metal “flanges” all connected to a center hexagonal, bolt like shape.
My only theory is some type of experimental gun-launched ATGM missle, much like the Israeli LAHAT or the more modern Russian Refleks ATGM. The flanges inside acting as a standoff for the missile??