that opening is the fresh air intake for the deluxe heater, see here - for the "lesser" 2 heater versions there was a "delete" plate held by 4 screws to cover the hole
"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