Many European gunsmiths modified Martini actions into target and hunting pieces, particularly during the pre-WW1 period. I have seen several German schutzen rifles built on Martini actions, a few shotguns, and even a paradox gun (German smiths in particular made the modifications to the trigger group and cocking levers like on your piece). Martinis were modified into shotguns by various armories for use by the Commonwealth Forces, typically for use by foragers and guards. Martini 12/14 gauge shotguns were also used, chambered for a cartridge with a 12 gauge head size and 14 gauge body size (these have a special prong on the bolt face to prevent firing regular ammo in them).
Look at the proofmarks on the gun. There will be the usual armory marks (typically British), but there may also be other proofs put on after the piece was modified that will give you some idea of when and where the action was modified. Common proofs are German, British, or Belgian.
