could be an interesting cape gun with a surprise chamber
http://egun.de/market/item.php?id=5030855#img
marked with Galand Paris & Birningham
left barrel 20 ga, right barrel 15mm rifled what could be anything from 28 or 24 ga with 40 or 65 mm long brass, 577 snider or 577 BPE is also possible
damascus barrels