I have some odd dies and a chamber cast for this, but sadly no cartridge. I posted about them on another forum and it was suggested that I bring it to the experts here. I was told that some folks here love a challenge. :)
The dies I acquired with a group of other dies and obviously came from an estate. They are RCBS custom dies, marked “412 M&V B.P. EXPRESS”. The assumption is that the cartridge was either a 10.3x60R or 450/400 2 3/8", in possibly a Miller & Val Greiss rifle. The chamber cast came with the dies, and I made a cerrosafe cast of the sizer die. Here is a photo of the chamber and sizer cast next to the COTW page for the 10.3x60R.
Cartridge length and neck diameter match pretty close for the 10.3mm, but the base diameter is smaller (.520 vs. .545) than what COTW shows as dimensions for either the 10.3x60R or the 450/400 2 3/8, the body appears shorter than the COTW photo, and the neck longer for the same OAL.
So, 450/400 and 10.3 are the closest rounds I can find, but not quite close enough? Someone went to a lot of expense to have some custom dies made. Any ideas?