Nice gun you got there, machinist ... expensive too

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Probably a
hunting escopeta, being half stocked; Fernando K knows better. Maybe he also has a record on Castaņo, who doesn't come in Lavin's work. Apparently this is only the lock maker, once the golden mark on barrel is signed by someone else, probably a more well know smith. The two letters you discern will most probably be those of Antonio, but this is a name rather popular and is not enough to ID the owner.
Maybe you have a friend with photograph facilities and post a close up picture of the mark; maybe Fernando K or other member will be ble to ID it.