Here are scans from "The British Shotgun, Volume one 1850-1870" by Ian Crudgington & David Baker. Although the Mathews top lever illustrated is shorter (earlier?) than the mystery gun's, both share a back set lever and the naughty bits of the locking bolt linkage are a dead ringer. Additionally the combination of high grade gun, short top lever spindle and single bite tend to point at an early top lever type.






Markus