Age Help Please

I believe these units are about 35 years old but can not confirm.
Electric Furnace Heat Wave S/N LO40423 Model # E1B35-23 who built Heat Wave?

AC is GE S/N 219370927
Any help would be greatly appreciated.

General Electric: Age is the last 3 digits of the serial number. First digit being the year
and the second and third digits being the month. Example xxxxx241 = 41st week of 1982,
92, 02 etc., xxxxx533 = 33rd week of 1985, 95, 05 etc…

i would say 1989

My records say:

GENERAL ELECTRIC [HEAT WAVE Furnaces] **“S” = 1980 **(go up or down from that)
Before 1975…last three numbers- …247 = 47th week of '62, '72, '82 etc…]

“L” is eight numbers down from 1980…making the date around 1972, which would be close to your guess of 35 years old.

But before '75 it was the last 3 numbers, in this case 423. How does March of 1923 strike you? I’d go with the 1975 possibilty if you think they’re 35 years old.

Thanks Wayne, and Jae.