So he would build parts by first making an oversize rough blank of bonded layers of marine grade plywood in a big press. Then he'd rough cut it various ways on a big band saw. Then he'd work his way through using all the sanders to slowly approach the net shape. He used precision squares to measure bigger stuff and calipers for smaller stuff.
I can't tell you the exact stability of marine grade plywood, but I know it's about as good as you can get for a wooden material, and I doubt he'd go to the effort of such precise measurements if it didn't matter.
