The 3/8" threaded hole has been drilled around the center of balance of the lens, it's a little bit in front of the diaphragma as the front element glasses are bigger than the rear ones, as a telephoto lens, it isn't a symmetrical formula.

The piece of bicycle inner tube (left down the lens) is absolutely not necessary : there's no contact between the lens and the aluminium profile, and it does not move at all, but for the first try...