Heavy Duty line in GMC Trucks was considered "500" series, & up. The "400" series trucks were considerd Heavy Medium Duty, & "300" series Medium Duty. Truck in question here is a 600 series COE. Chevrolet didn't have a Heavy Duty Line until the 1956 model "7000", & "9000" series Low Cab Forwards(LCF's), & 8000, & 10000 series conventionals came out.
