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.

1965

80,000Btu/h

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.