AFAIK J R Cobb was not aircrew in the RAF - he was A&SD branch (Admin and Special Duties Branch) so doubtful he rose much higher than Fl/Lt before he transferred to the ATA - where he was i believe a First Officer.He was a pre war private pilot.
From the London Gazette
1941. The undermentioned Pilot Officers are granted the war substantive rank of Flying Officer: — 9th Apr. 1941. John Rhodes COBB (81795).
Given the date of promotion to W/S F/O - I assume he joined the RAFVR very soon after war was declared,so probably did 3 or 4 years service before transferring to ATA (1943).
I have not thus far found any more London Gazette entries for him but I always find Gazette searches quite patchy.