This is only a guess but I think this is the 450 BLACK POWDER EXPRESS, with 120 grs of black powder, and useing a 270 gr bullet! Probably the same case as the 450NE 3.25" only an early black powder express!

450EX 120/270:

450EX= .458 dia Express round

120= 120 grs Black Powder

270= 270 gr bullet, probably paper patched lead!

Can you tell if this is a bottlenecked cartridge, or a streight case? If it is streight the above applies,and is the exact load listed in the 1910-11 Eley ammo catalog! IMO, this is the cartridge your rifle is chambered for!

If however, it is bottle necked, I may be the 500/450#1 Express also a black powder cartridge. This one is made by necking down the 500 case to 450, with a finished length of 2 3/4" case, and the load offered for it, in the old Eley catalog, is 450 bullet, with 110 grs black, and a 270 gr bullet!