Chudds

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