Even though spoilers were not available as an RPO (regular production order) on 1967 Camaros, they were certainly installed--and a lot earlier than many people think.
Here's a photo taken October 1967 of the 67Z prototype car. Photo appeared in Feb 1967 issue of Hot Rod magazine:
Invoice dated Jan 26/67 for a Gorries Chevrolet "Black Panther" Z28, calling for "spoiler":
Early calendar year 67 ad for Yenko Chevrolet Super Camaro, offering "Fiberglas rear deck with spoiler":
Another ad from Yenko, this time showing a rear photo of their 1967 Z28 "Stormer", complete with rear spoiler:
So, to answer the original poster's question, it's possible your spoiler was dealer installed, but not probable. The above are exceptions to the rule.