According to H&H the .500/450magnum express was loaded with..

1) a 325grain lead bullet 140grain blackpowder.
2) a 475grain leadbullet 125grain blackpowder.

Even though H&H call their 140grain load for a 5drammer it was 136,5grain.

Eley & Kynoch catalogs show loads with 325grain and 365grain as what I see. These were loaded between 1885-1919

When H&H took the .500/450 into cordite mode the `magnum` was removed. It was simply called .450 Cordite loaded with respectivly 370, 420 and 480grain bullets.

Eley however still call their .500/450 cordite ammunition for `.500/450 Cordite magnum`. Eley load this cartridge from 1900-1919.

The rifle in question which is marked 4½dram(122grain) would for me suggest it´s regulated for the cannelured 475grain