The color markings match the markings typically found on a GT front sway bar (has nothing to do with any other color marking combinations) though the diameter is not normal for the standard GT application. Its possible that the provider marked some sway bars incorrectly and they were shipped to the plant and installed since workers on the line would only look at the colors before slapping them in a car. Of course it can be difficult to measure a sway bar and things like rust and goop can add to the size if you didn't get it down to metal before you measure Just in case
Looking at 1968, Ford didn't list competition suspensions in their parts catalogs but showed a .858 as being the standard GT or improved handling front sway bar and the .958 as being installed in a K code 67 and GTs with the "maximum handling package" (what we refer to as the competition suspension in 67. 68 289 4V GT's used an even smaller front sway bar (from a page dated Feb 68)
Its a stretch - but a possibility IMHO. I would change it out though, as you did, since you were not the first owner and can be really sure its original to the car. By 73 allot of things were altered and changed on these cars especially when they were big block 4 speed GT's for example
And welcome to the site - yes we have a very narrow focus but its on purpose. Hope you find lots of things to help you in your projects