In the photo I posted 9X was No.2 Ferry Pool, which might explain why the aircraft is so clean. There also seems to be some fresh dope strips on the port wing where panels have been recently sealed by maintenance crews doing last minute work. The absence of a letter after the roundel would imply an aircraft that was not on a regular squadron and the 9 series includes a lot of MUs, station flights etc. where the aircraft would not be part of a squadron line up and so no individual aircraft letter would be needed.
It looks like the mystery aircraft is a station flight hack, or being used for fighter affiliation or some such dreary duty.