Finally got around to pulling emblems and also compared to Brian's pictures in
http://www.concoursmustang.com/forum/index.php?topic=5517.msg30998#msg30998I recall getting a service replacement from Ford back in the 80's (image on right) which appears to match what Phill posted in this thread and Brian in the other. The front of the emblem on the left appears to match the C5ZB version Brian shows however the back is clearly different.
1. Looking at the G the service replacement has more black in the area circled and the red ends in a rounded point instead of the sharper point on the left.
2. The top of the G on the left measures ~ .40" from the point shown. The replacement is ~.50 and the overall height of the GT alone is ~ 1.11" on both. When comparing with Brian's picture by zooming to identical sizes on an iPhone, the dimensions align for both left and right images.
3. The 5 points at the top of the Service replacement are rounded while the other is not. This also appears to be identical To Brian's 65 OEM piece
I can't confirm the emblem on the left was born with car but do know it's been with the car since at least 1978.