Judging by the shape of the buttstock, the decoration of the rear end of the barrel, the shape of the lockplate and the match holder, this is a late haquebut of ca. 1580. I cannnot properly read the date on the barrel but it should be from that period. 1420 is as impossible as 1530.
It would be great if you could get there and try to take a better picture of the date, or of the description which really should mention the date.