Mine is the same on my Mk IV Tim. I would suggest that they are also dimensionally the same but I can measure mine if you need it. It is made from 1" x 1/8" flat. I remember that the mounting holes are at precisely two feet centres which fits the dumb iron studs. The swan neck reverse curves are to bring the top of the plate below the bumper edge for full visibility. I don’t know what the two part numbers mean as you only need the one item as shown by Rob, above. The ‘mount’ to my mind is the bar shown and I do not know what the ‘bracket’ is. Maybe there was a short stay from the top of the bar to the bolt retaining the crank handle guide, as this bar is fairly flexible. There would need to be a central hole for that. Sometimes UK parts lists included the actual registration plate but those descriptions you note don’t seem to be that. I set out to make one and got in a real mess trying to form nice matching bends and gave up, resorting to reviving the bent, buckled, and rusty one. It can be done but you would need to have a frame that you can draw the bar around. It has to be done hot.
Just a note from experience, the registration plate is closer to the ground than you think and occasionally you can drag it on the kerb when doing a front-in 90 degree angle park.