As long-time readers should know, I’ve had ongoing research for a couple of decades about date coding in serial numbers for HVAC equipment and water heaters. As with political projections, I can now project that if you find a Gaffers & Sattler furnace with the following type of serial number:
B 20285E6
The B is the manufacturing plant,
the 20285 is a consecutive manufacturing number,
the E is the month (A = January), and
the 6 is the last digit of the decade, 99% of the time the 1970s.
You’ll find some sources that state that Gaffers & Sattler last manufactured as Gaffers & Sattler in 1970, and while that’s true to the best of my knowledge, the brand name lived on for some more years under the manufacturing auspices of Armstrong and Magic Chef, with some known units manufactured by General Electric (and possibly others) during management/union conflicts in the 1970s. General Electric units may or may not have the type of serial number shown, but the ones manufactured by Armstrong and Magic Chef should.