Methinks your well known stockmaker prefers to work with/sell high grade walnut stocks.
He makes them out of whatever wood the customer chooses and doesn't steer one way or the other. You can send your own blank.
When I contacted him about a plywood stock, I had assumed that the material would be stable in a wet environment. He wasn't trying to sell me walnut, he was being honest about plywood not being as stable as I'd hoped.
