Hi Timbo, An interesting question to be asking.
My curiosity has got the better of me, so i hit google.
I put in , " what does HIY/HIX, and H mean on post war vauxhall cars". And it came up with different pages to look at that explain. The best one was on http://vauxpedianet.uk2sitebuilder.com
This has a link to Vauxpedia which has loads of info about it. Might be worth looking at that to help understand the meanings of the afore mentioned lettering.
Regards Andy.