Verified answer

Leah,

you need to contact the Bureau of Labor Statistics. See this link (below) and on the left side it reads "Contact LPC" and they will have the information for you. This is a complex question and your focus could be centered on the Second World War where UPC was necessary for Aircraft, Tank, Higgins Boats, and other armaments that were necessary for successful production capabilities.

Contact them below and they will help you focus.

All the best,

Gerry