We had a new Endeavour 40 from this era in my family and I have also helped a guy who was fixing up a 43 that he had bought out of the charter trade. These were very mediocre (bordering on shoddy in terms of systems installation) boats in terms of build quality and the 43 is not a very good sailor. My recollection is that the decks were combination of balsa core and plywood (balsa being far preferable (for rot resistance) to plywood except at high load areas). I believe the hulls were un-cored.
Endeavour eventually upgraded their designs and build quality later in the company's history, but these predate that upgrade.