I emailed the Birmingham proof house some time back and asked what loads should be used in a steel barrelled hammergun with NITRO 1 1/8 proof marks from between 1904-25.

They replied:

"Dear Writer,
I am not sure where you are but guess you are sited in the U.S.A. so I will answer both ways namely:-
a. U.S.A. answer it can be used with standard velocity 2 1/2" nitro cartridges where the shot load does not exceed 1 1/8ozs and the packets made in the U.S.A. will be marked 2 1/2 dram equivalent.Must be lead shot.
b. G.B. 850 bar marks on box for 2 1/2" 65mm nitro cartridges standard velocity -1 1/8oz load of standard lead shot."

GDU