As you may know under the "old" registration numbering system of 3 letters/3 digits and latter when reversed ...
The 2nd & 3rd letters were Area Codes relating to counties and major boroughs.
All Scottish registrations were either *S* or **S
OR - anything with an S in the 2nd or 3rd place was first registered in Scotland.
Except for *SZ (Irish) - *ZS was never issued.
Similarly anything with an "I" as the 2nd or 3rd letter were registered in Ireland.
This was all hunky dory until the mid to late 1960's when the concept of "Cherished" numbers were "invented" and a whole business of personalised plates started up.
Previously a famous Conservative PM for Worcestershire acquired a whole fleet of cars with NAB 1 to NAB 8 for his family - he was Sir Gerald Nabarro.
Nice reg letter/numbers could be transferred to your new (posh) car and (what we call now) an age related number was assigned to the donor vehicle.
The DVLC used unused number allocations from mainly Scotland (the "S"s) and some of the smaller Welsh counties - e.g. Radnorshire with *FO
I remember a friend of mine acquired PAU 1 and close to us there was an E-type Jag with 1000 UP etc. etc.
This was fine until dealers started to transfer numbers to mopeds and lawn mowers.
The DVLC started to tighten up the rules.
And then the "prissy" letter/number combinations list was drawn up.
Let's not have a list of dubious registration numbers
I think we all know that 666 was not allowed - yet - 5EXY was put on a red Mini - I saw it!
So, clearly some dubious combinations slipped through the DVLA net, and this was before people started messing with the colour of the fixing screws and playing with the letter spacing - leading to more legislation
One thing to remember is not every reg number containing an "S" is on an age related plate.
Land Rover did sell some vehicles in Scotland (and Wales).
In the early 1960's they started to run out of the 3x3 format and they invented the year suffix and then the year prefix combinations and then we have the current abomination...
Summary of the
History of the UK car registration numbering systems