the lense axis "d" goes through the camera back. The small gear center is offset from it. The distance is the amount of offset from the axis line " D" measured at a right angle from "d" The image you show is a side view which doesn't show the measurement needed. A top view would be better.
So, you say, in a top view would be like this ? The distance among D and H ?
Don't know about the #28 but it's probably a manufacturing #, not the pitch number. Your pitch is 32
Yes, you're right: #28 is just a catalogue #. Amusing .....