Quoted from pinsrus:Ya not really sure the purpose of the non used lamp sockets but backlighting would make sense. I'm not sure the thinking behind using 2 different boards in production tho? What happen they got to number 543 and decided it wasn't a good idea.?
My guess would be:
They had a run of the prototype style boards made, then they decide to scratch the light bulbs to save money "it looks fine without them", and so when they put the traces down, they don't include the traces to the lights. Then when the first however many boards run out, they have the next run made without the slots for the extra bulbs, to make the board smaller, and hence cost lower.