The current format AB12 CDE did not appear until September 2001 therefore the first issue used the age identifier 51 (we have not had 01 yet).
There are 100 biannual age identifiers which can cover 50 years and 2050 would be allocated 50 and 00 (it is also feasible that 01 could be used for the March 2051 issue).
Registrations also consist of a 2-letter local memory tag and end with a group of 3 random letters. The letters I and Q are not used as elements of either of these groups (nor Z for the last letter of the local memory tag, nor J, T, U, X or Z for the first letter of the local memory tag).
There are also a few non-allocated local memory tags, which brings the total number of combinations for this 2-letter grouping to (19x23) - 3 = 434
therefore the total number of permutations for valid registrations is given by:
434 x 100 x 24x24x24 = 599,961,600
less any that may be weeded out if the resulting combination was deemed to be offensive or obscene.