[David_Hayward (RIP)]

Could your serial number be # 5 for 1945 Model, then 1311 etc.? I have no record of a 19XX series, but XX11 was Chassis with flat-face cowl, and 1311 was a 1/2-ton commercial chassis. There were also some sedan chassis assembled by GM-Holden's...Model 1016 for Master and 1216 for Master Deluxe.
It took me so long to find the GMC photo that you beat me to the draw! Yes, 1945 Model 1311 1/2-ton Commercial Chassis with flat-face cowl, sequential # 9 etc. However that#s only part of the story! The plate is a blank plate indicating that this was a chassis only, and that "P" indicates Perth Plant assembly making it rare. The Engine # is a 1945 unit, and the "CR" indicates light Commercial variant of the 216 Chevy unit, and Right-hand drive. However, as the Chassis number is stamped in Oshawa style, I would guess that it was shipped as a chassis minus engine and then the engine was shipped seperately in crates, and added in the Perth Plant who stamped the Plate. The body would have been built in Woodville Plant and then railed to Perth: Buckland Ave and Victoria St, Cottesloe Beach.

Of course whether GM-Holden's uprated it to 15-cwt for military purposes I have no idea..is there a weight limit plate anywhere?

Holden's also assembled GMC Utes, as per the attached though it was badged as a Pontiac...same plant, same engine, but different rad grille? This would also have been a 1/2-tonner.