Failed on number plates ? Clearly you have an MoT tester who doesn't know very much and as you say, pre '73 the rules are entirely different. Where I go for my MoTs they are both understanding about the quirks of older cars and also very willing to chat about what they look for, etc.
I'd be tempted to make an official complaint because the response you've had isn't being thorough or over cautious, it just indicates someone who hasn't bothered to read the rules about what he's testing. That is a bad mark for the profession and should be cleared up.
On the sill covers, they look very good, that decal looks great. I just have plain black vinyl on mine and something like that would really brighten it up. I've used "ding dong" myself for period stickers, they have done good work before so I think I might drop them an email. Thanks for the info.