To be pedantic, the Kings X - Hitchin part of the ECML, the Hertford Loop, the Northern City Line (Finsbury Park - Drayton Park), and Hitchin - Letchworth were all MkIIIa. The Braintree branch was also MkIIIa AIUI.
BedPan, the rest of the ECML, GEML from Colchester to Norwich, Carstairs - Waverley etc was all MkIIIb. The main change with IIIb (rather than IIIa) was the addition of bolted-base masts (rather than planted masts) to speed up and simplify construction. MkIIIc is identical to IIIb, but replaces the problematic AWAC catenary (prone to galvanic corrosion) with a copper catenary. Bronze is now used pretty much across the board.
MkIIId is mainly used for renewals and emergency repairs, particularly on the ECML, but Airdrie - Bathgate was done with MkIIId throughout from new.