Both, John. "Snake" covers came on some cars as standard equipment and many more purchased later at local dealerships. But it appears they were never offered as a separate, factory (RPO) option.

Also, if they were a factory item, what years were they used and how were these ordered from the dealer on a new vehicle.

As you might guess given their C9ZZ- p/n prefix, these trick covers first appeared during '69 production. And there's no doubt they were installed by the factory during 1969 and 1970, possibly more often in '70.

Their exact factory application, however, is unclear. Interestingly, promo artwork suggests they were to be included with Ram-air....I've seen no factory photo showing standard induction and "snake" covers on the same engine.

And this makes a certain amount of sense, vis-a-vis Ford's 1969 marketing terminology. For that year only, 428s with standard induction was marketed as the 428 Cobra. The addition of Ram-air made them  428 Cobra Jets.

Still, Ford's MPC makes no such distinction between applications. In every edition, they're listed as being correct for all 428CJs and SCJs, just like the plain aluminum covers. Change of plans? Maybe, but I can't be sure.

Hope this helps.

Mr F