EG Gem
With thanks to Steve. The owner says:
Gaggenau Ironworks (Eisenwerke Gaggenau) Gem type .177 smoothbore – Sn 4522 – Gun has the crossed Flurscheim air pistols so it was made from 1885-1898 (Theodor Bergmann) page 129 in Hiller’s 3rd Issue Rare gun marked PATENT