I always thought it was A, B, C on a saloon and A, B, C, D on an estate - as shown in Andy's pic.
Rimmers agrees with RAVE though and they both use A, B-C, D on a saloon and A, B-C, D, E on an estate.
So there are obviously two conventions. Great, just what we need - more confusion!
Here's the first convention:
link
Here's the second convention:
link and pic below
Btw, is it a pillar or a post.