Today I change a Lennox g8137 furnace> the damm thing had the original gas valve and other controls. Only the blower motor was changed.
Serial number was 66-12. Now this is well befor my time, havent even seen a g8 for 12 years and am not sure if the 66 is the year.

I also changed the Honneywell T90 stat with the lennox name on it.

Here is the catch. no visible cracks in the heat exchaner in the normal locations. If it was a g12 it would have been done.

Home owner opted for a 80% but its still going to save some decent money