XA890 was involved in Project 'E', and proving trails for rapid blooming window and infra red decoy flares, all prior to mid-1962. XA890 was transferred to RAE Bedford in 1964 for installation of experimental take-off directors. In 1968 she was transferred to A&AEE for further armament trials.
Perhaps the above time frames may assist you in determining the origin and nature of the markings.