Harrison [as in the radiators] is/was a division of GM ohwell is this the heater you mean? I know it was used for a bunch of years as the dealer supplied cheap version, and I've seen 2 Deco trucks with them, although might have been installed much later .... I'd put the vintage as "pre-1959", which is the latest year I've seen one in, but you might find a date code on the core

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics