Lovely old rifle, and complete with case and original tools!
I infer that this rifle is chambered for the .577/.500 Magnum 3 1/8" cartridge, which loads in Black Powder Express were typically 440 grain bullets. Nitro for Black loadings were the same 440 grain bullet with Eley also offering 480 grain and 570 grain loadings as well. The heavier bullets almost assuredly intended for single shot rifles.

I think the owner has misread the barrel engraving bullet weight due to softening of the script when the barrels were polished/restruck but not re engraved. A close look certainly looks to me like they originally read 440 grain with the middle "4" softened to almost a "1".

In that era where were no 650 grain loadings for the .577/.500 (not to be confused with .577 Express)

- Mike