My late Father, join date 01.04.1957, number L420****
It is not just a matter of when he enlisted, in 1957 we did not have the check character, it was a matter of if he was still serving at the time of the change.
Yes there were seperate groups of number depending how you started your service.
National Servicemen had two groups of numbers, one was used for former ATC members and the other for 'ordinary' National Servicemen.
There were two similar groups for Regular's.
There were the well known numbering scheme for Apprentice's and Boy Entrants.
With the latter, most of us could work out the Entry of anyone within +/- 1 Entry, just form his service number.