From the shape of the lion passant punch I think that this is probably a Birmingham mark. Unfortunately published Birmingham marks are a bit sparse and this one is not recorded.
However many Birmingham-registered marks were also registered at Chester (as Chester gave a much quicker turn-around than the overworked Birmingham assay office). Chester marks are comprehensively recorded and there is a very similar mark in my Chester reference registered to Rosenthal, Aronson & Co of Fore Street, London and Melbourne, Australia. They described themselves as “Australian merchants” and would probably have commissioned silver wares from Birmingham silversmiths and had their own mark stamped on the pieces as “sponsors”.
You aren’t by any chance in Australia yourself are you? If so that would pretty much confirm the identification.