Here is what I think is an interesting little piece of Mustang history. I recently picked up this emblem. It is made exactly like a 65-66 style GT Badge, yet, as you can see, it says "Grand Sport." This came from someone who worked for the company that made these prototype badges for the Mustangs.
There is a book called "Mustang: The Complete History of America's Pioneer Ponycar" that was published in 1979. It is written by Gary L. Witzenburg. This emblem is pictured on page 64 of this book along with some other prototype emblems. It is the only other place that I have heard of this particular emblem. Apparently, "Grand Sport" was just one of many names that were considered for a particular option package before the Mustang was built .
Here is a picture of an original production '65 Mustang GT Badge that most all readers are probably familiar with.
Anyway, if someone has any more information about this unique emblem, I would love to hear from you.