The case back appears to be deformed on the inside somehow. This is the inside of the case back for a ref. 35238000, which is for one version of the blued dialed Cal. 1151. Different case number (this is 1750084) but the inside of the case back is completely flat, and yours appears to be bulged inwards.
I just went through photos of about 30 of these snap on 7750 variant watches I've serviced and none of the case backs look like yours. That's not all of these that I've serviced, but it's enough to tell me the case back is the problem, so you likely need a new case back.
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