An internal photo print of said prototype was posted by the Facebook group, Ghostbusters Incorporated Worldwide. This proto Ecto-1 is based off the The Real Ghostbusters promo. It still looks cool, feels delicate and doesn’t appear it would fit a single figure, let alone three. Remember a Real Ghostbuster of your choosing usually sat in the gunner chair up on the roof of Ecto-1.
I wonder if internal photos of proto first series hero figures are out there.