c/o is thought to mean 'customer order', but I don't believe the Ford assembly plant would care one way or the other enough to mark a bucktag with information like that. The intent of the bucktag was to ID the car and options, but also to ID options that required some type of modification to the basic unibody.
13 is the welding bay
L4 is the date the car was bucked, November 4th
4 SPEED was on the buck tag since an additional hole was needed on the toe panel riser for the speedometer clip
HP for the choke cable hole in the firewall
PIO for the GT options, which included different dual exhaust reinforcements in the rear, plus holes in the rad support for fog light wiring. Maybe also includes the hole for the proportioning valve mount.