What about private postmarks?
Don’t know what that is— people cannot postmark their own letters as a valid USPS postmark.... afaik.
The hand cancelled postmark (date visible) at the P.O. those stampers is what should be required by Elections.
The way it could be manipulated is the assigned postmark date-— after the fact of 8 PM on Nov. 3rd. This was/is my point.