The difference between the 16:1 and 19:1 ratio boxes is in the worm gear. It's a matched set of shaft and block as the balls recirculate through the block back to the other end of the shaft and as a consequence the pitch of shaft and block needs to be the same. I never counted the grooves but the quicker ratio must have fewer per inch. I believe 19:1 is a rounded number and that ratio is actually 19+:1, so 8 for the quick box and 10 for the slow one makes sense.