Load-Master and Thrift-Master are actually engine references that correspond to model designations. A Load-Master being a 235 equipped truck and a Thrift-Master being a 216 engined truck. The Heavy-Duty 235, Option number 225 (different letter codes depending on model) was available in 4400 series trucks (1-1/2 ton nominal rating). The 4400 Series trucks are equipped with the 216 as standard in 1950.
https://www.flickr.com/photos/advance-design_parts_co/50255205127/in/album-72157631872034068/
https://www.flickr.com/photos/advance-design_parts_co/8250843887/in/album-72157632186114015/
https://www.flickr.com/photos/advance-design_parts_co/8250844293/in/album-72157632186114015/