Looks like three terminal capacitor.
That may be what is known as a feed-through capacitor. Usually used for filtering, the symbol would be used to indicate that it is constructed as a low inductance three terminal device. Different than I have usually seen them drawn, probably someone reused a MOSFET symbol rather than drawing the traditional feed-through cap symbol.
I found that Belton part number, it is a plastic combo jack, one of those things with a 1/4" connector in the middle of an XLR connector.
Near as I can tell the guitar isn't actually earth grounded
I think what happens is that when a plug is inserted, the sleeve part of the guitar plug connects to the S terminal in the connector, which disconnects that terminal from the signal labeled as IP_JACK and from the T terminal. The T terminal gets lifted from the switch contact when the plug is inserted, and that signal goes through the inductor L1 and to the input op-amp.
The S terminal goes through the other half of L1 (dual inductor package? Common mode choke? can't tell from symbol) then to ground through the feed-through terminal of C501. That signal going to the symbol that traditionally means "earth ground" but here is probably used for signal ground is capacitively coupled to the ring on the input connector, which I would assume is chassis.
So to sum up, after all the switch connections are moved around the guitar sleeve connection is connected to circuit ground through an LC filter, and that LC filter is referenced to chassis. So seems like a way to float the guitar ground from chassis at low frequency, but have RF filtering.
