The DVLA did indeed issue A suffix plates to any vehicle registered prior to 1963 when the original number was sold off in a cherished retention or sale. they did this right through the 80's and well into the 90's from memory.
I bought my 1959 S2 with just such an 'A' suffix plate on it back in 2006 and at that time I was able to write to the DVLA and get it transferred for a 'dateless' 3+3 registration. Interestingly, the new 3+3 plate was not issued under a 'non-transfer' basis so I was free to change it again.
Having found out the original number that was issued in 1959 as part of a V888 request (they gave you much more info then than they will now) I set about trying to track down who owned the original number. For me it was surprisingly easy, as the V888 request included the name of the woman who bought the number off the S2 back in 1991. It also included her address at the time.
Now banking on the idea that most people, may generally move house but many stay in the same area, I searched the phone directories for the area in which she lived at the time and started to systematically ring up the list of people in the area with her initial and surname.
I was pleasantly surprised that on my fifth or sixth attempt, I managed to find her and she still had the number on retention!
After a bit of explanation as to why I wanted the number she agreed to sell it to me at a reasonable sum.
So I now have the original number to put back on the S2 in about 15 years time when I've retired and have the time to restore it!
By then the DVLA will probably have banned the ability to do so, but if you are determined, it is not that difficult to find exactly who owns a number.
Any private detective would be able to do that for you in a very short time, for a fee of course.