They would have had a technical drawing of the top lid, which (if it was a mould drawing) would have shown the various ribs. Put a sheet of Vellum over the top,

They would have had a technical drawing of the top lid, which (if it was a mould drawing) would have shown the various ribs. Put a sheet of Vellum over the top, sign it, then use a graphics tablet (Summagraphics were around back then) to digitise it into a CAD workstation. Heck, if you're cheap, use two pots to make a two-piece digitiser arm and scan it that way (Mike Cook wrote an article in BBC Micro User back in the 80s about this). Measure the angles then do some geometry to figure out the Cartesian (X/Y) position of the arm. Create a point cloud, join the dots. Once you have the outline, creating GCode would be a relatively simple task (for a computer). Also I don't think the ribs are for support, though they will certainly do that. To me it looks like they're to allow the plastic to flow into the mould faster. Hunt down the "Engineer Guy" video on injection moulding if you're interested in the details. It's a fascinating process. Simple concept, but the devil is very much in the details.