Back in't good old days, the last two letters were representative of a geographical location. For instance, ANA and ANF were both Manchester numbers. If you take the last two letters, NA and NF in this case they would have been allocated to the Local Licencing Office in Manchester. So NA, NB, NC, ND, NE and NF were all Manchester numbers. The Lord Mayor of Manchester has the number N10 on his mayoral car which would have first been issued about 1903 (ish).
There seems to have been quite a bit of variation in the allocation of numbers in the early days as in some areas the 3 letter registrations were issued before there was a national system. For instance, the first car my Dad owned was a 1958 Isetta bubble car that had the number 856 KEN (a Bury plate. In 1962 he bought a new Austin A40 which carried the plate 8416 NF. So Bury issued three figure licence numbers before Manchester.
In January 1963, the first suffix was issued in the January of that year so for example ABC 123 would become ABC 123 A. The problem with this was that the motor trade complained that most of their sales would take place in December for delivery in January and therefore they did next to no business for about 11 months of the year and went berserk for one. This was because most drivers wanted a car in January because a 1963 car registered in January was, effectively a new car for longer than one registered later in the year. Because of this the issue of the new suffix moved to August. Some people wanted to be the first with the new letter while others like to buy a new car in January so they can have a current year motor for longer which had the effect of spreading sales over a longer period.
So in 1965 (or 6) the government of the day put the new system into effect. Another aspect of the numbering system was that certain letters weren't allowed, such as I because it could be mistaken for 1; Z because it can look like a number 2 and Q for the simple reason that it was very similar to the letter O or number 0. I and Z though were used in Ireland.
New car dealers used to be allocate a series of numbers which were usually released to dealers a while before August and they used to be entered in a ledger in adming or the sales manager's office. When a car was sold the salesman would simply put the details of the car and its buyers next to the appropriate number. People could choose, within reason, which number they got. When I bought my Uno in 1989, I chose F636JMA because 636 was about as distinctive as they had on offer. Although batches of registrations were supposed to be allocated to dealers on a random basis, a hell of a lot of Volvo 142s, 144s and 145s had single or double digit numbers rather than 3 figure numbers that most cars had.
Once we got to the letter Y, it was all change to a prefix, for instance instead of ABC123Y, we had A123ABC. When they ran out we ended up with the present system.
In the case that MEP mentioned, it's quite possible that his local Ford dealer registered both cars as demonstrators. The two numbers would have been one above the other in the ledger so both would have been registered on the same day, or at the very least within a few days.
So, the last two letters were geographic either by county or town and would have been issued more or less sequentially