Fiberglass was not the least bit rare in the 60s; Corvette was fiberglass from day 1 in 1953. The factory-built 64 Ford Fairlane Thunderbolt drag cars had fiberglass doors, hood, fenders, some front bumpers, etc. Many fiberglass body parts back then.
You got told what I believe is one of those "back in the day" type stories that mixed and matched a bunch of BS.
First, Gibb took the first 50 of the 69 COPO 9560 ZL-1 cars, but later sent some back. These cars all were delivered with the steel ZL-2 "cowl induction" hood.
Second, of those first 50, there were 13 Fathom Green cars http://www.camaros.org/9560list.shtml
Third, what is pictured above is known as an L-88 hood, so named because the style first appeared as an option (or part of the L-88 option) on the L-88 427 1968 corvette. The ZL-1 Corvette did use the same style hood, with ZL-1 badging, but I have never heard or seen in print that style called anything but L-88.
Your hood above appears to be Rallye Green vs Fathom. Could just be the flash lighting.
Now, the L-88 hood was used on Motion Performance built 69 Camaros (and 67, 68, 2nd gens, Vegas... Probably a choice on just about anything Motion could sell. There is a famous Rallye Green 69 Motion Camaro (google "Mexican camaro") that had an L-88 hood on it, but it had a single white stripe in the center. And the hood pins were in different position http://www.carguychronicles.com/2011...in-motion.html
Green Meanie 69 Camaro wore an L-88 hood green with white stripes.
Those straight areas can't be hollow; the hood hinges bolt into those areas on the back of the hood sides, right? Don't usually see anyone trying to bolt into plain fiberglass, let alone hollow fiberglass.
I would tend to agree it is an early aftermarket hood, if for no reason other than the quality of the underside. Unless someone more knowledgeable about the dealer converted cars weighs in here on whether the above hood looks like a certain type/mfg used by a certain dealer, I think your ad is just going to be for an old school molded fiberglass hood.