... IF its a headless screw, I have a hard time visualizing how this would
stop anything from turning.
The typical screw does have a sort of ‘head’ - - - perhaps a millimetre or
slightly less of non-threading at the top so the screw will stop before it
disappears into the threaded hole. So the tightened screw sticks out by a
millimetre or so. When you centre the shutter on the lens board, that screw
will hit the board and prevent the shutter from sitting flat against the board.
You file or drill is small notch or hole for the screw so that the shutter
sits flat on the board and the little screw is “captured” in the notch.
You can also tighten the jam nut pretty well by hand without the anti-rotation
screw: remove the rear lens element if it sticks out very far or is larger
than the jam nut. Then screw on the jam nut and tighten it by
hand-and-fingernail with the lens slightly counterclockwise (viewed from front)
from where you want it. Then hold the nut and board with one hand and with the
other hand slowly turn the shutter clockwise. With a little experimentation
you can achieved acceptable tightness without a wrench. Of course, this works
only if you do not have that little anti-rotation screw attached!
