[Army Air Corps]

Also, the moulds from that era were milled out of steel and were a major investment for the model companies (I have read about “average sized” moulds costing $150,000). The moulds are still expensive, but a lot of them are electrically milled copper or copper alloy (according to the model biz publications).

[Abathar]

It’s called EDM machining, you machine an electrode out of either copper or graphite and charge it in a tank of dielectric fluid and it arcs between the electrode and mold, disintegrating the mold. Takes a while, but it about the only way to get sharp internal corners in the mold without inserts, and on tiny pieces thats not an option.