Denny,
All 47-55 trucks without a heater from the factory came with a cover (and seal/gasket) over the hole on the passenger side. By 1949, all trucks came from the factory without a heater (and there was no factory RPO for a heater). That plate was, of course, still needed when the more common recirculating heater was installed. I think I have a couple of those cover plates, if you want me to post a photo.
Tim
