Frigidaire used fiberglass/mineral wool. They didn’t skimp on it either. The wiring is all asbestos though. Great stuff. Doesn’t burn and short out. I don’t worry about the wiring it’s encased in a sealer.

I have a 56 Frigidaire imperial, this ones big brother. The oven is built and insulated just like a modern electric stove. It seems to cycle less and cook more evenly than the newer POS it replaced.