Undoubtedly they are mould numbers, so if there are problems, especially within the factory, they can quickly be tracked back to the relevent machine. Imagine for instance that a whole bunch of legs turn up with the left boot not properly formed - they can trace back to the machine and mould for the blockage, or perhaps the plastic wasn't hot/fluid enough.
We've had a couple of discussions about the moulding processes, and the flow and rapid cooling of hot plastic is an interesting problem in fluid dynamics, and something my wife know an infinite amount more about than I do.
I've got a couple of very old Knight klickies on my desk, chromed bodies, non-turning hands and chassis the same colour as the legs, and they lack the mould numbers.