It was not that rare in the beginning of fiberglass boat construction. Here's a thread of the subject of several uncored deck sailboats.I have confirmed what I was told by the seller that on my 1966 Hinterhoeller, both hull and deck are uncored. I am curious what the implications are and how common it was for early fiberglass boats to be completely fiberglass?
Red Jacket is the first cored hull. Cored decks were common prior to that. The Pearson Triton, introduced in 1959, had balsa cored decks.The first cored, fiberglass boat is generally believed to be "Red Jacket" - a 40 ft, 1966 C&C.
So it would be very unlikely that your boat, built in 1966 would also have a core so soon after this.
(Now I am open to corrections if I'm wrong!)