Thermal Couple I can't find an explanation of the powerpile... but the one thing is that a thermocouple has a metal tube, and a powerpile has wires ... I think the output of a powerpile is higher than a thermocouple ... but I admittedly ain't a gas guy ...
I'm pretty sure that switch is your AUTO/MANUAL switch.
I don't believe there's a pressure sensor on the aquastat.
There MAY be one elsewhere on the boiler though.
I'm sure the L model is obsolete ... probably for years ... I would imagine if needed, it could be replaced with the J model.
so, what did you do to fix ? apparently the expansion tank needed service for starters ...