20 foot Master Angler floor repairs-rebuild

[lunasea17]

I began with some exploratory holes in the deck in suspect areas and was suprised to find plywood core with glass over and under. I thought all of these boats used end grain balsa core? Boat is a 1976. Could this be a replacement deck with the old skin reattached. If so it was done with no obvious joints that I can see. It's in much better shape than I expected and I may be able to just replace some of it. Thanks in advance for any in put.