Stamped $2 bills from the bank cause reader to scratch his head

An Independent reader brought four $2 bills into our office last Thursday, Aug. 1, with a question.
He said he likes to get nice, crisp $2 bills from the bank to give out as tips after meals because the waitresses think it’s fake money. It isn’t, he said. The $2 bills are worth $2.
But on Thursday, he said he went to the bank for $2 bills and they gave him these four that each had a 13-cent postage stamp pasted on it and they were canceled with a hand stamp in Dearborn and Belleville on April 13, 1976. One of the postage stamps was a Bicentennial stamp, two stamps had U.S. flags on them, and one was in honor of “Chemistry.”
They are all still good to spend as $2 bills, but he wonders what is the story behind all this.
If anyone has a clue, call us at the Independent so we can share it with our readers. Come in or call (734) 699-9020.