In attempting to answer my own question, I found from two sources, "Cartridges of the World," and "Cartridge Conversions," that the .375-2 1/2" has to be made from .405 Win. brass. Not as easy to find as .303, but available (again).

I remember seeing a Remington-Lee sporter in .405 once also. Gotta be a rare bird.

Anxious to here how your pretty Mannlicher performs!