Plywood is good for dimensional stability, but I'm pretty sure all this work must have been done in and around a toolroom with stable moisture content or the part was used immediately and then consumed/destroyed before it "moves" too much. However, he sounds like he's pretty knowledgeable so I'm going to guess this isn't just garage woodworking where 1/32" doesn't really matter when the wood is going to shrink/expand by 5-10% over the course of a year.
So the patterns once done would be taken to a foundry where they'd be used to make molds. I'm not totally up to speed on that process but I know it involves surrounding the pattern with a combination of refractory sand and binder. Where it gets tricky is complex parts that have multiple cores and so on.
And yeah, this guy was retired at the time but he'd been doing it for like 50 years. I'm very sure he knew what did and did not matter.
