I've been through the 56 sqdn record books and I think I've solved the problem. The serial of the SE5a '3' in the photo is partially obscured by the struts and is, I believe, actually C1086. This was Junor's aeroplane and it was first flown by him, for a 'test' on 27 March. He then flew it until he was killed in it on 23 April.
The composition of the Flights in March and April 1918 was not as static as it had been. In addition to pilots flying Special Missions and flying in pairs, it appears that people were flying in mixed formations, under different Flt Commanders or ad hoc leaders. But Junor flew mainly under Galley, who was B Flight Commander. So I think that when he first flew C1086 he was a member of B Flight, hence the 3 and the wheel cover colours.
Note. Burden flew D283 to 2 ASD on 28 March and while there collected C1096 and flew it back to the squadron. The two SEs were therefore issued to the sqdn only a day apart and I don't think there would have been time or reason to have marked the flt markings and then changed them.
C1086 went to B Flight and Junor and C1096 went to C Flight and Burden.