It looks like someone welded a lid and a turret on to a 1960s/70s vintage DUCK.
;-)
That “1960s/70s vintage DUCK” you remember was actually a 1943-45 vintage DUKW. A six-wheel drive amphibous truck designed by GM. And as for their rarity, I drove past one yesterday afternoon on 14th Street in the center of Washington, DC. It was driving to its passanger pickup point to get folks for a tour of the Potomac River.