What is a core-worker?
"A core is a device used in casting and moulding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in die casting and injection moulding. For example, cores define multiple passages inside cast engine blocks." - Wikipedia
(That was a better description than I would have managed.)The people constructing the cores for the moulds would probably have worked closely with the patternmakers who worked at each foundry.