Ted Hooton did a load of work on Spitfire camouflage schemes in the late '70s/early 80s. I have somewhere at home an article that resulted.
He listed several serial batches from Supermarine where the A-even/B-odd rule of thumb was reversed. .
For Castle Bromwich built aircraft, for some unknown reason, the A/B scheme serial rule alternated between batches.
The application of the A/B scheme ended after serial number X4912 for Supermarine aircraft and serial number P7810 for Castle Bromwich built aircraft. Manufacturers were asked to complete aircraft in just one scheme and all seem to have opted for the A scheme. Hooton wrote that this occurred in December 1940, which is entirely possible. The modification may have been entered into the Supermarine ledgers sometime later.
Don't forget that the rule of thumb should be reversed for Hurricanes!
Cheers
Steve