On Sat, 14 Dec 2013 07:52:40 -0800 (PST), Ian
wrote:

I am installing a new NEST thermostat, replacing an old two wire mercury bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

Can anyone tell me which letters on the NEST base (W, Y, O/B, AUX, E, G, C, etc) they will map to? It has to be the most basic of installations; open or close the circuit.

I don't believe a Nest will work on your system. Did you do fill out
their compatibility form?