I have a J&W Tolley set up for the NA&A 50/450 3-1/2" Magnum BPE.

First I'd ever heard of that cartridge too.
Looks like Marraki's Tolley might have been set up for that one too.

The 50/450 magnum is very similar to the 450#2 NE, except that it precedes it by fifteen or so years and has a thinner rim and more powder room.
Turn the rim thinner on a 450#2 NE case and it'll chamber.
OAL is somewhere in the 4.300" range depending on what bullet I'm going to use, haven't regulated this rifle yet.

I'm using virgin 450#2 brass as donor brass to work with for my gun.