ChrisV289,
Are you sure your car had rear seatbelts?
At some point, the car had front and rear seat belts. There are factory style seat belt mounting points in my Fastback for the front and rear seats. Unless deleted or removed, the 65 Fastback came with front seat belts from the factory and the rear seat belts were an option. It's not clear to me whether the rear seat belts were a factory and/or a dealer only option. Maybe someone can shed some light on this? In any case, I figured I would install the seat belts so when I went on an occasional drive, the occupants would at least have a lap belt.
GT500KR,
Here are some cores on eBay right now. My 64 1/2 April Dearborn car used the painted buckles like these. (Red in my case).
http://www.ebay.com/itm/64-65-Mustang-Seat-Belts-/380669711039?pt=Vintage_Car_Truck_Parts_Accessories&hash=item58a1aed2bf&vxp=mtr
Thank you, thank you and thank you!

It was not clear to me the exact style seat belt buckle that would have been in my September 2, 1964 Dearborn built fastback . I figured it was not the plastic (?) clam shell type found later (after 1/65?) in the 65 model year. Several points of reference (books, web, etc.) kept indicating that with a build date of September 2, 1964 my Mustang would have had the 64 1/2 style seat belt buckle. I had no clear indication of whether the early 65 seat belt buckles in cars such as mine were all Chrome or painted color coordinated. Based on your response and the eBay link you provided, I'm pretty confident I would need the 64 1/2 style buckle, painted black if I want to keep my Mustang as original as possible. In the short term, I may have to settle for some sort of "classic reproduction" belt as these things are hard to find let alone very expensive to buy and restore.
Are your seat belts for your 64 1/2 considered a standard seat belt or deluxe? If yours are like the ones in the eBay link, They sure look like the 1965 "deluxe" seat belt. Take a look at this link from Python Restorations.
http://www.pythonrestoration.com/Ford_Belts.php This is one of the websites that contributed to my confusion.