Hmm. Yes. This goes way back to 1988 when I first acquired the vehicle and had no money, and little care for attention to detail. Sometimes this is a good thing in terms of just "doing things", rather than worrying about small technicalities.
The bumper was swapped in from an an old REME S2 vehicle that (sadly in retrospect) probably never drove again. My original bumper was twisted all over, so it was a good swap.
The number plate is the 1970's or 1980's standard square white one with black plastic sticky-out letters on. It was fitted by the PO (to the original shape?) and was fully visible behind the original bumper. (When did the yellow/white plastic reg plates come in? I am assuming in 1960 the plates were different?) It doesn't really fit behind the military bumper and the bottom of the lower letters are very slightly obscured if you were too look at it from absolutely horizontal. However, between 1988 and today I never once got pulled for it, or failed an MOT.
You'll see that I also changed the plate to silver-on-black. I had no money. The black paint is just Wickes' version of "Smootherite" and the silver is Airfix gloss silver G8. At the time it was just a cheap way of doing it and I expected to get comments at MOT but never did. I went over it a couple of years ago and it actually looks really good. When the sun catches it at the right angle the 3D nature of the sticky-out plastic letters, and the silver paint looks really cool (well I think).
So, various levels of marginal non-compliance if you want to get technical.