There's no possibility it could be an early prototype is there?
It might account for why it has some features but lacks others that maybe came in later, perhaps they were just trying brass out as a material to use?
No idea, I'm intrigued by the fact it has a main screw retaining plate that has the hole for QR lever, the main nut is mounted in pretty much the same way as the QR half nuts are, and the number 5 on the side for the model number is unlike the markings I've seen on any of the other Parkinsons vices I have. Also the use of VICE rather than VISE.
Seems strange to only be half brass though, and having a steel bar through the steel main screw, surely that's the most likely part to have an impact?If I understand correctly, brass tools of all types were used where needed in munition factories, to limit the chance of sparking an explosion ?