Change of NINO

I'm researching into the war record of my grandfather, James Conlon Warburton who died in 1978. I have a copy of his employment record which states he had two National Insurance numbers, one commencing ZH 11 37, which was deleted in his record in favour a new number commencing TY 50 68. Are you able to explain why as it was my understanding that your NINO, once issued stayed with you for life. Thank you

Hi,

Unfortunately, we are unable to explain how your grandfather was issued with two National Insurance numbers. Your understanding is correct, once your National Insurance number is issued, it stays with you for life. 

Thank you.