The very first double rifle I ever bought, back in the early 1960s, was a Cogswell & Harrison .375 2.5", same type as you have there, and it also was the one and only double rifle that I ever sold too (actually, made a part payment trade with it, on a Westley Richards that I still have). That original double, for me, came with a ton of Norma .303 brass, that was longer than for .303, so was used to form .375 2.5" brass (I still have and use that brass). Most recently, I noticed, surprisingly, at least four of these same type C & H doubles, in same .375 caliber, for sale on market, so about a month ago, I bought one, my second C&H that is identical to yours too. Additionally, I have three other British double rifles in this same caliber, one a hammer gun that is regulated for 300 grain bullets, while all others I presently have, including C&H, are regulated for 270 grain bullet, thus I am sure that yours is also regulated for 270 grain bullets (velocity is around 2000 fps, while pressure with this cartridge is quite low). I have handloaded this cartridge for years, and shot many deer with it.
By the way, why form your own brass, when good .375 2.5" is readily available in U.S.A. today? Best powder for this cartridge is IMR 4895. This was first .375 developed by British, called .375, 2.5 inch, followed up years later with the .375 mag., flanged and belted versions, for double rifles and for bolt rifles.