Hi The Black Car,
Are you hoping a higher price will make you feel better about your purchased? IIRC, I think I have seen asking prices as high as $800 or so. That does not mean they sold, but asking. The emblem you have is more generic and not specific to GT350 or GT500. That cap will work on the first two 66's as well on the 67 wood wheels. It will not work on the wheels that were first offered in 1968.
As an aside, there is a real nice repop of that cap out for about $200 with emblems. The below image is from Virginia Classic Mustang. Brant offers three different emblems, maybe a forth. The 66 COBRA emblem had two versions, one with gold and one silver band. I could only find the one with gold band. It is believed the gold band came on the first version of the 66 wood wheels and the silver on the second 66 version. But it likely they also could have been intermixed. So it is not an absolute. All the 67 horn buttons have the silver band.
In addition, if anyone has an original horn button missing parts or damage to pieces, the repop makes for a nice donor for replacing missing or damaged parts.
There is also a COUGAR cap for the 67 style wood wheel from West Coast Classic Cougars.
Take care
Cory