LFFC
The spec aircrew system was slightly different. A spec aircerew Flt Lt promoted to Sqn Ldr before the age of 45 would automatically transfer back to the single list (the CS in todays terms). This was to be avoided at all costs as you ended up with a Flt Cdrs workload with very little prospect of further promotion (since you were competing with the young 'high fliers' who were promoted to Sqn Ldr in their early thirties).
A spec aircrew Flt Lt promoted to Sqn Ldr after age 45 remained spec aircrew. This was ideal since as a pilot you were pretty much guaranteed to keep flying to 55 and amongst other things you were no longer eligible for orderly officer.
You're right about the CS Sqn Ldr, denied PA Spine, watching in dismay as his PA Spine Flt Lt deputy is promoted to PA Spine Sqn Ldr. Especially grim when he realises that his deputy is now lined up for a ground based Gp Capt pension at 55.
Flo