In the bottom of Cherry and “direct” clone switches you’ll see four holes there: Gateron red for example
The inner two with the round holes are our normal in-switch LED holes. The outer two with square holes would have been used either for diodes as shown above or jumpers. You’ll also notice the diode symbol by the holes; this is most likely a more modern addition but I’ve no vintage switches on hand to look at, and it could be used to reference either plain diodes or LEDs since they are light-emitting-diodes.
Jumpers and in-switch diodes would have been used back in the day of more complicated/pricey PCB manufacturing leading to the usage of simpler/cheaper single-sided PCBs that would have required the jumpers in order to make the matrix cross over itself. Diodes could achieve the same purpose but also add in anti-ghosting. I can’t say for sure if the diodes might have come first with the jumpers just being the cheaper version. Nowadays we’re using bare minimum dual-layer PCBs that eliminate the need for wire jumpers and instead can just route on both sides of the board with the occasional jumps between layers with vias. Some fancier stuff may utilize more than just two layers but I’m not aware of any boards that do.
