That case could very well be a “swing ring” type. No case back to remove. Remove the bezel and crystal, and look at the edge of the opening in the case for a place to put a fingernail, at the 6:00 position. If this is a swing ring case, the crown will have to be pulled outward as in setting the hands. The movement has a rim around it, the rim being hinged to the case at the 12:00. Carefully, lift the movement on its hinge. Maybe get back to us with a picture of what you see.
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