Poked around a little in regard to the .375-2 1/2" and found an article on a very similar wildcat described by Ken Waters as the ".375 Express." He explains its creation and feeding in a 1997 "Handloader" article. Case made from .444 Marlin and about .25" shorter than 2.5".
You might have a look at that article re handloads appropriate to your Mannlicher. It is reprinted on pp. 1026-28 of the collection "Pet Loads" by Wolfe Publications. If you can't find it easily and want the dope, PM me and I'll try to get good copies of the pages from "Pet Loads." Usually I lose part of the page when I try to copy that big, thick book, but suspect I can get you the gist. If I can't I'll look thru my pile of "Handloaders" and see if I have that issue.
Looks like a great "medium rifle."
