Yes, but....what? ;-)

I see your point, but are you sure the fender is original?  And when you say "invoice", does this refer to a sheet (in window sticker format) generated by Dearborn, or something on the dealer's letterhead?  If the dealer sold a radio from his own stock, then anything's possible.  Also, look to see if the antenna cable extension is a Genuine Ford part - the dealer kit came with a standard cable, only.

I have also seen some other 71 stangs with the antenna on the rear fender.

To date, I've seen three vintage Mustangs on 4 x 4 chassis.  I would not be surprised to find there are more, yet I seriously doubt any of them left the factory in that condition.   ;-)

Also when I entered the International Mustang Meet last year, I was not penalized points for having it installed on the rear fender, I was deducted for the wrong radiator cap, the wrong oil filter, battery cables and the wrong nut on the air cleaner lid.

As with the above sightings, this fact is interesting but hardly constitutes credible evidence.  There's no guarantee a club's rules will reflect anything more than the wishes & beliefs of the rulemakers.  They are independent bodies, rarely forthcoming about the sources for their info....and their rulebooks aren't exactly factory-approved.

This is not to say clubs don't have their place - clearly they do.  But it would be circular reasoning to suggest their very existence lends credence to their beliefs.  And, please, don't get me started on any specifics or its bound to hurt someone's feelings.  For now, I'm content to have laissez faire: they don't tell me what's correct and I don't tell them how to spend all those thousands in fees.   :-)
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Mr F