14 ga so a No 3 Collath caliber http://bazar.hunting-shop.cz/inzerat/457683-prodam-brokovnici-hamerlesku-14-70/

W.Collath Söhne combination gun 16/70 +5,6x50R,SN 19031 + 6488?
https://www.egun.de/market/item.php?id=12742653

clearly restocked, the bigger rifle barrel (9,3?) got a liner in 5,6x50R so it can be the reason for the second SN