The KEW Files which hold the Japanese Index Cards are from WO 345/01 to WO 345/598. Unfortunately, they are not in alphabetical order which makes it hard when researching.
For the last three weeks, I have compiled a database which details which names are in which Index Card KEW files. I have added it to the Research page in the FEPOW Family as others might find it useful.
A further step would be to add the PoWs service number but as there are 58,000 names I have added it would take too long to broaden:-
Hope it is found useful.