you realise it is possible to mail unsealed, open envelopes, right? doesn't seem that tricky...
You put postage on the letter, and the post office stamps the postage (to invalidate the postage). The stamp contains a date. You can't stamp something after having mailed it, that happens as part of the mail submission.
What confused me was how you would achieve post office stamping over a seal that's on the wrong side of the envelope, where the flap is.