The /1 always means it is a half size Bigger than the gauge ahead of the /. Nominal 16 is 662", 16/1 is .669". Up until actual diameters appeared this was normally checked with a series of plug gauges. Thus a bore marked 16 would accept the .662" plug but not the 669" one. A 16/1 marked bore would accept the 669" plug but not a 677" (15 gauge) plug. A 17/1 would be a half size under.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra