T = Temporary. WS = Wartime Substantive
As you suspect, he holds the rank for the post that he's in at a particular time; chances are, though, that he'll not have reverted to lower rank at any point and to outsiders will appear to have been promoted steadily from Sqn Ldr at the start of hostilites to Air Cdre by 1944.
He's a substantive Sqn Ldr in 1942. He's then promoted to Temporary Wg Cdr (presumably to fill an SO1's slot in Bomber Command HQ). He continues to be paid as a Sqn Ldr.
Then, presumably on appointment as the Air Attache in Moscow, he becomes a Grp Capt (the usual rank of the Air Attache) - but continues to 'enjoy' the pay and allowances of a Sqn Ldr (his actual substantive rank). This is partly rectified with a back-dated promotion to substabtive Wg Cdr in November 1942 (note not back-dated to his appointment as Air Attache, but at a guess to the date he arrived in Moscow to assume his new post).
Then, on 20 Jul 44, he becomes Chief of A2 at SEAC. This is a 1* post, so he becomes a temporary Air Cdre - but in receipt of a Wg Cdr's pay and allowances.
Finally, in Jan 45, he's given promotion to the substantive rank of Grp Capt and receives the pay and allowances associated with that rank - but as this is a wartime substantive rank, once the war is at an end (remember that bureaucracy will have not felt any need to take into account the surrenders of Germany and Japan when determining when wartime ends...), he reverts to being a substantive Wing Commander and his pay goes down again.
I'm guessing that there was a point in 1945/46 when he was doing a 1* job and getting a Wg Cdr's pay.
Somewhere in the Treasury, a beancounter chances upon this thread and has a bright idea...