"IF" it had been between .662" & .669" it would have had the 16 in circle for nominal chamber & 16 for bore size. The 16/1 means it accepted a .669" gage but not a .677" one. .677" is 15 gauge, 16/1 is halfway between 16 & 15 dropped to three decimal places, thus .669". The bores were marked with the largest plug gage they would accept. In calculating the whole number gauge sizes they were "Rounded" to three decimal places but the "Tweens" were simply a fractional part with all digits beyond three decimal places dropped.
