bill wrote:
" I am assuming it goes to a relay
box first which closes the
circuit to activate the aquastat/boiler and circulator. Is that where
the wire goes, to some kind of relay box? "

What I mean is the end switch terminals closes a circuit. If it does go
to the circulator, then it must go to a relay box first because the end
switch is 24V, and the circulator is 110V, so the relay box simply
takes the 24V completed circuit from the end switch to pull in a relay
which in turn activates the circulator.
However you might have a different setup. The end switch might just
complete the circuitry for the boiler to start up. In this case it is
24V.