I believe the fractional marks on the "BARREL FLATS" are 13/1. 13/1 = .719" in diameter. This means that when the barrel received its first proof the bore would accept a .719" plug gage to a depth of 9" but would not accept a .729" (12ga) plug.
A 15/1 mark would put the bore between .685" & .693", highly unlikely for a gun chambered for 12ga shells.
Interesting that Wonko, got "Up on his Tricycle" so big in trying to correct the term "Watertable" yet made no mention of the fact that was not even where the mark was located at all. To describe the barrel flats as action flats is just as inaccurate as saying watertable. So there.
