The 90s (and Mk III DVTs) were built to replace all the class 81-85s that were still in service in the late 80s. When ordered, the 90s were classified 87/2, although they were 90s by the time they entered service.
Mk III coaching stock was first introduced in 1974 on the WCML; there was another build (Mk IIIb) in the mid-80s to replace the remaining non-air conditioned stock on the WCML. Most of this is still in use on the London-Norwich route today.
To be honest, as far as the passenger was concerned, the only difference between the air conditioned MkII stock and the MkIIIs was that the MkIIs were 3m shorter and had tread brakes which made quite a noise when used. This was of course helpful as it woke you up when approaching your stop, particularly on a Friday after a few shandies.