On my 86 I believe its regular plywood with Teak veneer outside and what appears to be Mahogany veneer inside.
I think on the 70's it was all Mahogany veneer. No need for marine plywood unless its in bilge or cockpit, imo.
-Karl
1986, 32' Sedan, twin 360ci, 275hp Chrysler's w/ K&N flame arrestors