You can use google with the site keyword - e.g. google for "pig site:hmvf.co.uk" -that restricts google to just find responses from a particular site. (handy on a lot of websites that don't have search functions )
Alas it doesn't get away from the OP's problem of finding all the vehicle descriptives in people's signatures. I removed them from my signature block a while back after similar problems.
At a guess I'd say 1943. 1942 sounds a bit early to me, and I can see a 1943 one on a quick google search.
There's a definite switchover point that you can see on CMP vehicles where the POW can holder changed to a universal type that could hold the old style cans and jerricans depending on what was around.
Comet and early Cent manuals list it as Key, universal, filler caps and trap doors. Part no. Z3035
Cromwell vehicle training pamphlet calls it Tool, combination, (supersedes "key, trap-door catch" "key, drain plug" and "key, petrol oil and water cap").
Gotta say, I got a bit worried when I saw a description of painting it in "number 2 brown". Not sure if it's a stores numbering, description of colour or a specification of what to use.
Maybe for the wider world, but in the UK a fair number would have survived. You can see a lot more CMPs, WOTs and Bedfords in UK civilian use in post war film footage than you can Dodge WCs and jimmies.
I'd guess that we see a lot more US kit at the shows because they have good parts availability and are easier to keep on the road.