.375/.388 Chatfield Taylor case, ie: .338 (or 7mm Rem mag) necked up, or .458 Mag necked down, by the sounds of it.

Hell of a good ctg. Easiest is to neck up .338 Win. brass.
J.J. Donnelly's "The Handloader's Manual of Cartridge Conversions"
ISBN #'s
cloth bound: 0-88317-138-4
spiral bound: 0-88317-137-6
paperback: 0-88317-136-8

page 465

The only discrepancies are minute:
Neck length listed as .370"
Shoulder dia. .490"
The only data I have at my fingertips is the single Donnelly's load:
62.0gr. IMR4064 - 300gr. @ 2,463fps

This appears a moderately light to moderate load, considering this is an 88gr. water capacity case.

This is the average speed a .375H&H gets with factory Winchester 300gr. ST loads from a 24" to 25" bl., so if it is accurate, it is a very good load, indeed with no real reason to exceed it.

However, the very similar Mike Epstein (I'm sure no related) Magnum, a 2.6" case with 1/10" longer neck of .464" lists loads in excess of 3,000fps with 270gr. bullets & 2,800fps with 300gr. Noslers. barrel is 26" Star Premium 12" twist WW cases (shortened .375H&H)to bet 2.6" length. Due to the longer neck, the actual capacity should be identical to the .375/.338 Chatfield Taylor.

reference:
pages 507 to 510. These loads appear WAY too hot.

Wolf Publishing Wildcat Cartridges Combo Edition 1992
ISBN 1-879356-18-X