XVin NumberXX 63R 17L 578
B 5A AC PB
FM QM
BTRAC DE
S-2 A
Kevin Marti published a book - The Mustang and Cougar Tagbook that explains some information related to this tag and allot about the others found through out the cars. A great reference book if you don't have one
The ones at odd (different angles) are thought to be inspectors or processes
Decoding can be a challenge since codes changed every year and even multiple times in a years as well as differing depending on the assembly plant.
First two lines related to information found on earlier cars on their door tags (projected build date, color, trans ) as well as the bodies first rotation number (your case 578) and notations to the body crew that the car was being built with air conditioning and power brakes
BTRAC is painted exterior racing mirrors (Mach I mirrors)
FM - may be reference to a stereo (need for holes for door speakers)
QM - Have no idea
DE - Possible Deluxe exterior but not enough sames to be sure
Hope this helps