The information available as a Primary Source, like the Registers, but can be contradictory, difficult to read and/or incomplete.
There are two Registers - one Alphabetic by initial letter of Surname and ranked by date of joining. And the other by date of joining. The service numbers between the two do not quite tally -- in other words a man can have one number on one Register and a slightly different one on the other. The quality of the information deteriorates with time, one senses an "end of term" feeling, and paperwork got put to the bottom of the list of things to do