When there is only one size marking then it is the bore. The word choke tells you it had some amount of choke but does not say how much. If you had two marks such as 16.7 & 16.0 then you would know it had .7mm of choke when proofed. 16.7 incidentally converts to .6575" or about .0045" under nominal. Under older "British" proof this would have been marked as a 17/1.
I do not know why but early L C Smith 16 gauges guns were bored to .650" so this slight undersize will present no problems. In spite of what you may read or hear otherwise it will not cause it to "Kick" harder.
