Marstrom Core Materials that I know of:

In 2002 the A-Class and M18 the hull and beams are solid carbon. The boards are hollow core. The mast has some kind of closed cell foam at various places. Repairing thin solid core carbon is tricky. It's thin and you have to taper or bevel the repairs.

I think the M20 is the same as the A

Tornado uses a honeycomb Nomex PAPER core. I spent a lot of money once to buy some NOMEX HONEYCOMB and found it was not the same stuff.