https://www.shopfesler.com/flush-mou...-10-truck-kit/
I don't see 64-66 but they make 67-72 and the square bodies also. Maybe what you saw was from the same company and was sort of a "prototype"?
I once saw in a magazine, this look was accomplished by welding sheet metal on the pillars to meet up with the stock glass and the glass was mounted with butyl tape. I dont really remember the details but that was the gist of it.