The first versions had exposed pads under the breadboards that made contact with the power rails. The voltage for each breadboard rail could be set with removeable jumpers, rather needing jumper wires.
The breadboards needed to be tightened down evenly to make contact and it was overall more trouble that it was worth so the idea was scrapped but the mounting holes were never removed. I figured they would make swapping the breadboards easier than using the adhesive backing.
This is genius! I wish I would have done that. If I have to change them out, they'll go back on with Velcro.
I have four testing setups now, but the Protoboard's set up for pots has moved it into most-favored status. There have already been times when I could have used hot-swappable boards.