The only way to know for sure is to e-mail Four Winns and see if they get back to you with the information. I'd doubt that they were using fiberglass stringers then, most production boats didn't yet at that time...they are probably either regular dimensional lumber or 3/4 in thick plywood...mine were plain dimensional lumber (not ply) and were not even 'glassed all the way up the sides, and not on the top at all. I had a major rot repair on this boat that involved repairing the stringers and putting in a new deck from the helm seats all the way back to the engine compartment. The quality of the glass work on the wood was pretty bad in my opinion.....I know now they use a no wood system that won't rot...but you still need a good bond between whatever they use for a core (structural foam usually) and the 'glass....and still want to keep water out of any penetrations you make in the deck or transom...water can freeze and delaminate the core from the 'glass...