I am not so sure that the word Jap was used as a racial slur at the time... it was just a shortened name as is Brit for British... however they were referred to as 'Nips' which was with reference to them being sons of Nippon.. and this was used as an offensive slur.
We 'Brits' referred to the Americans as Yanks during the war but it was not an offensive offering nor was 'Frog' or 'eyetie' ...... I guess I feel some sadness that out society is becoming too steeped in modern 'awareness' to see a nickname or shortened name as lighthearted.....