I've had a few radio delete a-bodies (70 and newer though) and they had a "plug" that went into the hole in the fender. I would check a parts book for your year car and see if there are 2 different #s listed for a passenger side fender. On my 70 and back b-bodies, the fenders don't have the hole at all and are a different part #.

Restoring a car to look "only" as good as the factory made it is like getting a boob job for your wife and having the doctor make them two different sizes to seem more natural