Hello Richard,
I'm still in the British/Canadian WW2 vehicle mindset of weight categories 8cwt, 15cwt, 30cwt, 60cwt, and US counterparts being 1/4 ton, 1/2 ton, etc, etc, which I need to get out of for this project. What you say makes sense, in WW1 there were less weight categories, but each category was much wider. This would explain lorries identical to mine being described as both 30 and 40cwt, but Im sure Ive seen photos of Napiers with the more intricate Y shaped cast wheels described as 45 cwt; but then it could be the photo caption was wrong or as you say it was purely a manufacturers classification.