Sam,
The two crowns in the last photo (IMG 0008) confirm Raimey's thought that the Drilling is a Sauer product. The Crown S on any barrel means it was proofed for shot, a crown W means it was choked( amount of constriction not shown), the crown U means the drilling underwent a view proof( a detailed inspection and confirmation of dimensions, after definitive proof). The 16 in a circle means it had a normal 16 ga. chamber, which was 2 1/2- 2 9/16" at the time( actually it still is, even though most all guns now have 2 3/4" chambers). The 16/1 is the bore diameter ahead of the chamber, this is about 16.99 mm.
Mike
Last edited by Der Ami; 10/23/19 06:15 PM.