Hello OHP 900, I bought all the letters from these guys
http://www.numberplates-4-u.co.uk/
He also made me up a "pressed" plate with 3.5" letters for my 4-door, whose original Hills plates were looking the worse for wear/time. He normally takes a stand at the Beaulieu Auto Jumbles.
It was quite difficult to get all the 3.5" letters and the profile of the last one to get (the P) was different to the B I already had.
There were a number of "type faces" and made in both metal and plastic. I could have had the correct shaped P, but that was an original aluminium one (a Hills one) and the price was much higher.
I'm including a photo of the rear of the metal plate to show that, because of the mix of manufacturers, the posts are different diameters. The metal plate was salvaged from my very first Traveller (before it was towed to the scrap heap in 1984) - reg 6717 PJ - but all the letters had been removed in the past and a "modern" plate placed over the top.
I thought that, at least, the P had an existing home (2 holes) but the replacement P had 3 posts![frame]

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The P is 2nd from the left and you can see the original top hole for it, only the bottom post fitted and I had to drill 2 new holes. You can also see all the other holes that did not match the letter/number posts.
The "9" fitted a treat in the place the plate's original leading "6" was.

'50 Low-light with 918 Side-valve engine,
'51 High-light with Side-valve 918 engine,
'55 4-dr with 803 engine,
'56 Traveller with 1098 engine.