I recall cored boats in my prime boating years (kids growing up). Kingfisher for one (I had a 15' BB, 70 Rude) and there were others in the 18' and shorter boats arena in my boating circles. Cores were Balsa Wood, roughly ¾" thick. I think they served 2 purposes on the Kingfisher primarily, flotation and hull strength without conventional bracing...like stringers. I do recall seeing at least one cored boat, forget the occasion, may have been my Kingfisher, and the core stopped several inches before the transom.
Lighter hulls of the day were glass matt/woven roving and chopper gun glass with a resin glue. More expensive, heavier boats were big on matting and you could tell what was used by just looking at the interior of the hull. Mats only showed up in little squares in a checked pattern. Chopper gun filler was little straight pieces of 1/64" (estimate) glass arranged at random. A couple of examples of chopper and mats were a '71 Chrysler Sport Fury for the chopper and 18' Caravelle bow rider for the mat. I would expect that after the gell coat (in the mold) at least one layer of mat would be the next item.
I wouldn't worry about buying an older cored boat if balsa and if you have a cored boat you need to check if it does go all the way to the transom if you plan on installing a deptfinder transducer at the transom, inside the boat...like I prefer. Solid glass transmits the signals fine, but mounting on a core would severely attenuate the signal. No biggie determining the performance:
Get it a quiet cove and with a Glad bag full of water, drop your transducer over the transom at hull depth and take a reading. Bring the bag full of water and the ducer into the boat, where you want to mount it, set it on the hull and take a second reading. Compare the results for your answer.
