I fix these all the time. I use a product called"Plastix" available from Eastwood. The black kit allows for almost an undetectable repair. It is an Acrylic powder.
I've done it several times with great success. To be effective, make sure you have enough of the colored disc.
Like he said don't move on to the next grade disc until the scratch is not seen. It needs 5" hook and loop pad on the DA.
I bought Ford service parts for my 65 back in 1983, they were not marked on the top edge like early models were on the original fenders. I did date stamp them to match production fenders. They did have stamped date codes on the fenders inner support near the door post. My 71 with original...
The inspectors had the odd shaped hand punches, they would punch as the assembly passed their station. It's possible to decode your paper build sheet and at least one Mustang restoration book that has many of the codes.
I bought PPG locally and removed their logo prior to installing. Had a Carlite stamp made by Office Depot. Installed it myself. After removing the old one, nothing is difficult.