The dust cover on a Moonwatch doesn't just keep out dust, it actually keeps the movement located since there are no case clamps, the movement, which is surprisingly small at 27mm dia, sits in a holder ring which is held in place by the pressure from the dust cover. The dimple/nipple on the 'dust cover' presses on the middle of the caseback. Or so is my understanding.
Get a dust cover, any will do, even a modern service part is better than none.
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