There were at least three types of heaters used in the 47-55.1 trucks. The one called the Fresh Air Heater had the motor housing protruding from the back side. This is what went through the big hole in the fire wall. Models that used other heaters had a block off plate covering that hole. The fresh air heater mounted to the inner cowl panel and drew air in through the louvers on the right side outer cowl.

"That is all I will say about that" - forrest gump