I seem to have gotten lucky, twice, because I found this a couple years ago, lost it almost immediately, and re-found it last night.
The note says, “Martini-Henry Cartridge Mk II for Morris Aiming Tube”,
and I wonder if that IS the correct name for the cartridge, rather than just what it goes with? Definitely one of the coolest rounds I have!
It measures appx: .875"/22.22mm OAL, .344"/8.73mm Rim, .223"/5.67mm Bulllet Dia, .239"/6.07mm Neck Dia, .277"/7.05mm Neck Dia., no markings anywhere.