I believe it is Koa but if I was you I would wet it down. Take picture of it and google Koa slabs and compare it to your picture.
However if it is koa you have a chunk of wood that depending on how much figure it has it about 300 to $500. Let alone it being seasoned for 35 years and it being one of the Islands favorite woods for ukukeles.
Why would you want to just make three lap guitars?
Tunnel vision. Suggestions? Right now I'm thinking of at least making a baritone out of it.