It appears to me to be solid hand laid glass, no coring.

Yup

You can drill into those areas to make sure they are wet/rotted. It's a ton of work to fix, make sure the boat is worth it to YOU. Do tons of research, make sure it's RIGHT. It also rotted for a reason, you need to fix that also. I used Nidacore for my stringers-it's a polypropylene honeycomb it will not absorb water ever. use epoxy, the secondary bond is much better.