I'd bet that most of the cars that were delivered to the dealer with no radio had an aftermarket radio installed before they were sold. I bought a box full of radio delete plates in the mid 1980's from a local radio shop that did (and still does) radio work for several of the local ford dealers. The owner told me that back then they could install an aftermarket radio for 10$ - 15$ less than ford charged and would send a tech out to the dealerships 2 =3 times a month to do just that. I ended up with about 80 Ford radio delete plates (about 20 were mustang..) and about a dozen or so incomplete radio installation kits for mustangs, most missing just the radio. Unfortunately I sold most of the stuff off in the early 1990's at carlisle
I'm guessing that 90% of the cars that left the dealership with no radio all had radios within 6 months or so, so almost none of the plates survived. 65-66 plates aren't too hard to find, early 67 plates turn up occcasionally, late 67-68 plates are tough, and 69-70 standard plates are really hard to find. The 69-70 woodgrain plates are nearly impossible (I've only seen 2 in 30 years of collecting parts..) As for 71-73, they seem to be scarce, but don't bring the money that the others do...