I believe the part number for the bumper is 358159-A2-S which is documented in the Mustang Assembly Manuals as follows:
1968 - Body Manual, AM0021 pg 20 item B, used on the ash try
1971-72 - Body Manual, AM0041 pg 27 item 2, front license plate bumper (as 358159-A25)
1971-72 - Body Manual, AM0041 pg 139 item 5, rear license plate bumper (as 3581598-2S)
1973 - Body Manual, AM0046 pg 41 item d, front license plate bumper (no rear drawing available)
Unfortunately I could not find a drawing depicting this part nor a reference in the Ford Car Parts (ref p/n 40544).
You will find this part's application in the Library Section of this forum under Mustang Hardware Spreadsheet.
I think that if you went to a Ford Dealer to get a replacement they would substitute "something".
Jim