18.4 would be the diameter of the bore in mm's. This converts to 0.7244". Working from memory here but I believe the chokes were not marked after 1924. If it is marked there will be another number similar to the 18.4 but smaller giving the choke diameter. If it were marked for instance 17.9 right & 17.4 left then it would have .5mm (.02") & 1mm (.04") constriction or for that day Half & Full chokes.

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