Component on case bottom of 48S

[KenW]

What is that large (~ 0.75 in dia) round component on the inside bottom half of the case of the HP48? A picture of the component can be seen in the photo bottom.jpg from here: https://www.hpcalc.org/details/4701?

I am curious because my 48S has/is suffering from a power-on problem which requires me to apply pressure on the case just above the 'B' key. Most of the comments I have seen assert that this issue is due to poor contact between the keyboard connector and the main PCB, so I disassembled my 48, cleaned the contacts, and reassembled and the problem was reduced in that I now need to apply less pressure but must still apply some pressure.

But, I did notice that there is a spring contact which reaches down from the PCB to this 'button' and that the pressure for turning the calc on must be applied in an area that happens to be right above the location of the spring contact between the PCB and this button. So, is the pressure being applied to aid in turning the calculator on helping to aid in contact between the keyboard and the PCB, or to aid in contact between the 'button' and the PCB?

Thanks

Ken