oh, wow wild theories here, it is definitely not a part of the mercury switch. here are the pictures of the mercury switch.
but what is it than?
i will tell you: beaniac idea goes in the right way. it is not a tax for espresso machines, but a tax and certificate
for pressure tanks aka boilers with a pressure higher than atmospheric pressure.
you will only find them on machines that were installed and running in italy. i have a 1 group and a two group e61 factory sold to belgium, they dont have it. i have a 3 group e61 i bought it in italy it has it.
so how do i know that?
in the 80th our company imported air compressors from italy to germany maybe 3.000 a year. we got them in a special color to brand them for the german market and they were made to order from the company in bologna. sometimes they could not deliver and we accepted their standard model with their color made for the italian market. Those all came with exactly this little round disc installed at the pressure tank. so i asked them what is this for? and they told me that this is required by the state to put it on each pressure tank to show that the tank has a proof and test certificate and the fees are paid. of course with the EU those things dissapeared.
hope this clarifies the mystery