The earliest production frame that used carbon fiber, to my knowledge, was the Exxon Graftek. It was produced around 1976. Bicycle Guide magazine did a fine article on it.
The article does mention that some prototypes of carbon fiber bikes were built prior to this, such as Raleigh.

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The Graftek does show up at vintage bike shows now and then too....



I don't think that there is any question about it being accepted as suitably vintage.

Steve in Peoria