To be honest with you I have not heard of this before. I served from 67 - 94 and had not come across this in my time. I will do some searching to find out, but it is a new one on me.
Official Numbers are sequentlal, and computerised as of the 70s onwards. I joined with P102453, then it went to P102453L, then to D102453L. Gone are the days of P for Pompey, D for Devonport etc. all begin with D.
Regards, Chris