The buck tag date, in your case Sunday July 10th, is generally considered the build date of the car. The buck was assembled on July 10th. Looks like Metuchen was working weekends to keep up with production orders. Because of the length of the assembly line, and when the buck tag may have actually been generated (could have been early morning or late afternoon), it's possible that the vehicle didn't roll off the end of the Trim & Chassis line until the next day.
As Charles says, the Warranty Plate on the door was "most likely different". It could have been before the buck date or after. For instance, my Warranty Plate and Build Sheet say May 10, 1966 while my Buck Tag says June 21st! This is probably one of the more extreme cases, usually the dates are a few days, to a week apart. The buck date can also be before the scheduled build date.
The Mustang Production Guide (a registry of Mustangs) might give you a "general idea" of "scheduled" build dates as related to adjoining VIN numbers, but you have to be careful, even here, since there are times when the dates and VIN's aren't sequential. This can be due to things like part shortages or if the car was a pre-sold unit.
The GT (Performance Image Option) code on the Buck Tag of
PI or
PIO seems to be used rather randomly at Metuchen thru the 1967 model year. When Dearborn started using Buck Tags in 1967, they used a
GT code. Metuchen followed suit in 1968 and started using
GT as well.
You may also have radiator support markings as is illustrated here:
http://www.concoursmustang.com/forum/index.php?topic=5222.0I've been trying to do comparative analysis of the buck tag, build sheet, and radiator support markings on a vehicle. The buck tag identifies the gate (stall or welding bay) that the car was bucked in. I haven't seen many Metuchen radiator support markings, but Dearborn used a system of marking the skid that the buck traveled on for part of the Body & Paint line. This is represented by the
F 507 on the radiator support in the above link I cited. I'd like to see if Metuchen also marked the skid numbers.
My theory is when the cars began bucking they didn't start out on the skid. They were built up in a welding bay using assorted sub-assemblies until a rough skeleton existed. Once this skeleton took a particular form of completion, the buck was then loaded onto the skid. The radiator support markings were added at that time when the early buck was mated to the skid, since the skid number couldn't be identified for certain prior to that point. This left a period of time in the early buck assembly in which a build sheet was probably used to develop the early features and options of the car. (again, my picture reference in the above link). Because of the sparks flying during welding, I can imagine that a paper build sheet saw rough use in a welding bay. This may have lead to the development of the buck tag, first introduced at Metuchen. The tag was generated much earlier in the buck's development, and actually identified the welding bay - long before the buck was loaded onto a skid. Bucking features could then be noted on a durable metal tag that was affixed to the early buck.
Whatever the reason, the tag caught on and Dearborn began using them in 1967, and San Jose in 1969.
