Need help decoding lennox htr age

[Mike Richards]

Hi, need help with age of horizontal lennox furnace, can't find it on any internet charts.
m/n gs 1513/4x-80-3
s/n 2325168724
house built in 1988

Thanks,
Mike Richards

[Ken Rowe]

I don't think you copied down the serial number correctly. It should be in a format like this:
5187G 24232

51 would be the Lennox division number
87 would be the year manufactured
G would be the month code
24232 would be the serial number

Do you have a picture?