An iron moulder made moulds (or molds) from which castings were made.
The moulder used a special wooden pattern or wax model to shape the mould. The model was then removed (or lost) leaving the shape required and creating the mould.
Note on spelling differences:
Most words ending in an unstressed –our in British English (colour, flavour, honour, neighbour, rumour, labour) end in –or in American English (color, flavor, honor, neighbor, rumor, labor).