Interesting,I bought a martini actioned rifle believing it to be a 577 black powder.
Research showed that it might be a 20/577 .
I showed it to a very reputable gunsmith recently and he seems to think that is a 577 Snyder.
His belief was that before the Brits addopted the 577/450 they also trialed the 577 Snyder.
Mine is very roughly finished and apart from proof marks has no other markings to identify it.

Marrakai is the man to talk to as iff my memory serves me correctly as he owns one..

Al