I've been trying to figure out just what these old dies are, curiosity as much as anything else. I posted about them some time ago, maybe a year, got some good ideas, then they went on the back shelf and I forgot about them.
They're 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. There was a chamber cast that came with the dies, and I finally got around to making a cerrosafe cast of the sizer die. Attached 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.
I didn't see anything in the book that matches dimensions. It looks like someone went to considerable expense to have some custom dies made. Any ideas? Anyone have either a 450/400 or 10.3mm round that you could measure the body length to the neck and the base diameter for comparison?