I don't think Olive saw the conversation that transpired, but I'm almost certain that this is the cause of something I have a hard time explaining otherwise.
John's regiment and serial number is listed on my father's birth certificate along with the other 'usual' details. This is what I suspect is that registrar's doing, because it's certainly not usually included on a birth certificate.
In theory, John would thus always have been traceable by my father.
In theory. 17/?