The 170 is 17.0mm for bore dia. This converts to .669" or +.007" over nominal .662" for 16ga. Choke simply means it has/had some degree of choke, but amount is unspecified. During some periods the Belgian proof house did mark choke size, but in those cases it will be marked with two sets of numbers. If it had for instance been marked Choke 16.3/17.0 then you would know it had .7mm (.028") of choke constriction.

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