Earlier HP41 full-nuts fixed their processors to the keyboard circuit by means of nuts screwed around the screw posts. The screwing allowed it to press the processor through the zebra to the keyboard circuit. The back side of thecalculator was designed that way: there was a flat side on the underside of the screw posts - see right side of the picture.
Later in production, someone decided that these nuts could ve spared by building a couple of spacers in the back side. These spacers would pass the pressure onto the circuit. You can see them in the left side of the picture.
The problem with this approach is that it made the tension od the screws fundamental for the proper working of the machine. With the screw posts under tension, there were more and more failures as they broke more often. Also, with the original design, the pressure in the lower part of the case was not required for proper operation - only the upper part needed to be secured.
But the first approach (the "nutted" one) was not free from problems either. You could unscrew it once, to make a repair; but it was very difficult to screw it back on the post. If you had to screw it and unscrew it a couple of times, you broke the screw post completely!
So we decided to replicate the newer back on the older back, by putting aspacer, initially glued to the back side. That had a problem: which size of spacer? there are different thicknesses of circuits, the earlier being thicker than the latter.
Then we realized we did not need to glue it: it was enough to lay it on the circuit, and we could produce it in a variety of thicknesses and we could use whichever fitted best in each case: