I've seen .577 Snider mentioned.

Also .577 Eley. I don't know what that is.

One website discussion mentioned three lengths based on the .577 Snider case.

But one must be very careful about internet discussions and sites because some just repeat stuff and make up stuff and opinion as fact. One discussion mentioned "tens of thousands of howdah pistol images" all " .577 case pistols". Now dudes looking up NitroExpress.com might believe there are huge numbers of howdah images floating around the internet, but that is simply not true. Else point the to me please so we can collect them. One of these discussions. Two of the participants are simply making up stuff, opinion and conjecture as fact. No doubt others will repeat some of this bullshit.