Harpoon launcher cartridge

I have this harpoon launcher cartridge,but I don’t know its denomination

the case is 50 mm long and inside neck diameter is about 10 mm 12,07 mm outside).Rim diam is the same as the 30-06, 11,90 mm
There’s a small copper cup that close the case.
No hds,the case is machined alluminium,belted and rebated.It has a very conical shape too