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You appear to be missing the inner anti-magnetic cover. This cover goes over the movement spacer, and is held down by making contacty wioth the case back - this arrangement keeps the watch firmly in place. Since that cover is missing, someone has tried to add case clamps (which are not used on this model) to make up for it.
Those should be removed and the inner cover installed, and the movement will be held solidly in the case.
Greetings, and welcome to the Forum!
The movement is really in need for a service, and I do hope that you have a watchmaker who knows how to work on vintage Speedys?
Please do share more pictures of the watch?
And you have an exciting serial nr....😀
Why is it exciting? Can you elucidate for those of us that have not memorized Speedmaster 101?