The gun should have a 16 in a circle as well as the 17. The circled 16 is nominal gauge for which the gun is chambered, but has nothing to do with exact bore size. The 17 mark denotes actual dia of the bores at time of proof. To the best of my knowledge as long as these gauge marks were used rather than an actual dia, marked in mm's this was taken with a series of plug gages. the 17 mark thus means a 17 ga plug (.649") would enter the bbl for a distance of 22cm (8.66") but a 17/1 (.656") would not. Nominal 16ga size is .662" in diameter.
