I have an old mechanical (mercury switch) lennox thermostat driving a heat pump - I am trying to replace with a Honeywell RTH7500D. The wiring is confusing as the Lennox matches nothing in the manual. I found a matrix on here that seemed to explain the Lennox thermostat quite well and when I measure voltages for heat, emer heat, cool, fan, etc it seems to be correct. The terminals are as shown below with the parentheses noting my understanding (so far) of each one. There is one terminal with NO label. It's voltage levels appear to match those of the emergency heat terminal exactly (i.e. it is 24 v when emerg heat is on). The terminals are noted below. Anyone know what the blank goes to and what it should be connected to on the Honeywell?

R (cool); V (hot); F (fan blower); M (heat pump compressor); BLANK (no idea); E (emergency heat); X (common)

Old lennox thermostat exact model number?

Looks to be 3AAT86B38B1 -- does that sound like a viable model number?

Old lennox thermostat exact model number?

Looks to be 3AAT86B38B1