[EdHolland]

OK Steve, here is what is hidden in those mysterious cans:

The screws securing the trimmer to the top of the cans run into the brass bosses seen here:

   

These are the trimmer screws:

   

And the two plates of the trimmer, sandwiching a piece of mica are underneath:

   

Removing the trimmer screw all the way won't cause a mechanical explosion:

   



As can be seen from the last picture, the screw is floating, electrically speaking. Any scratchiness (dirt on the threads or in the brass insert in the ceramic) is only a factor when you're connected to the set while making the adjustment.

Hope that is helpful,

Ed