Before their could ever be a The Real Ghostbusters version of ECTO-1 from Kenner, their had to be a prototype and approval from guys in casual apparel.
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.
Sources:
Ecto-Containment 