You do not show the marking which gave bore size. Realize that choke is amount of constriction not an actual size so these "Choke Gauges" are mostly worthless unless you know for an absolute certainty the bores are to exact standard size, which very often they are not. Nominal 16 gauge diameter is .662" which converts to 16.8 mm. If your bores run smaller than 16.8mm then your choke diameters also will run smaller. As Raimey said best bet is to pattern them & see what you get.

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