I always try and incorporate a year of make coin in my cars somewhere, glued to review or vent window corner, the 41 I used a 41 quarter as a bezel under the headlight knob.
When I worked for Buick in the early 80s I found all kind of strange things. Pepsi cans, marbles in defroster ducts, beer bottles under rear seats and carpets but my favorite was the nut tied on fishing line in the cowl on both sides. It drove me nucking futs!
I've used copper 1 cent pieces to block holes in radiators. Too lazy to cut out a piece of copper sheet. Before 1966, We had real pennies. Big copper ones which you could solder to.
I say use it in a shift knob, or laminate it on the dash. If you put it back, it's just a penny under the carpet (they minted 1,098,201,100 1956-D cents). If you display it, it becomes part of the car's history...and a great story. As long as the penny stays with the car, you might as well make the best of it.
I tried... like I said.... my boss is a cheap skate.. Last raise was 8 years ago... Lately it seems the older I get, the harder I gotta work.....