Holly-General furnace age

Looking for the age of a Holly-General Duo-Trol 80VF furnace Serial# 18341.

I’ve looked at all the charts I could find, none of them even mention Holly-General, Holly Forced Air or the Siegler Corp.

The house was built in 1960 and the unit looks like it could be original, but would like to be sure.



Thanks Les!
How did you arrive at this? I can’t find any info or charts to use the serial #.
House was built in 1960, not likely that the unit was replaced in 1965.