Hey Nico, check this video. If you skip to 3:00, you'll get an exact explanation of the nature of these bolts.
The short story is this, they are used to attach the fretboard to the superstructure of the instrument as these were built as 2 separate pieces. An adhesive would be applied to the main structure, and then the fretboard attached. To ensure a tight bond, the two pieces were bolted at 4 places along the back of the fretboard.
You will see 2 screws towards the base of the instrument as well. These screws held the bridge in place as the bridge was also a separate piece. From what I have heard of the Polycarb, it offered some valuable learning lessons for what would become of the Railboard. I've always that the Polycarb had something going in the left hand that made it especially good for the bass work. They're special.
