You can sometimes work them out but you need multiple sources

I once worked out a code because in the Log Book of a pilot he recorded the Codes and the ORB recorded the codes of the aircraft he flew. But by process of elimination, comparing codes with known fates and serials etc I got down to one possibility

This was not absolute proof but then aircraft was recorded as having an accident of a certain day, I know that the accident card recorded the serial number and that the possible ‘code’ aircraft had an accident the same day (the only one listed in the ORB and same circumstances) – It was not 100% proof but good enough – I think it would have held up in court

But you need multiple sources to do this and then eliminate

Sometimes you get lucky ORB has Serial Log Book has Code and if you know P/O X only flew “A” on date DD-MM-YYYY you have a match

Also you can sometimes work them out based on history of aircraft movements, if Aircraft “A” Serial XXXXX was lost on certain date and two days later a new aircraft turned up but was record in ORB as ‘A’ as well but with Serial YYYYY then you can make a good partial guess that YYYYY was ‘A’ as well