I first encountered the expression in the services. It may be, if the expression originated in the services, that the reference was originally to the the person's service number.
I have heard (British) policemen saying such things as, "Let's call in 346; he lives in that area," rather than, "Let's call in PC Bloggs", which might support my idea.
But it's only an idea while we are waiting for a response from someone who knows.
Student or Learner