Although early Mk.IXs had a short(ish) carburettor intake, it was different from the standard Mk.V version, being wider, and with a bigger mouth. The longer tropical/temperate intake was introduced 29-12-43, but wasn't made retrospective.
The so-called "single blister" cannon-bay door was not introduced until 7-1-44, but was made retrospective.
Extended-horn elevators were introduced 13-9-43, serial nos having no bearing on which a/c had them, and which didn't. The change was also issued as a leaflet-style modification, so Squadrons could do it, themselves (having been made aware of its impending arrival, 29-3-43), and retro-fit extant airframes.
The pointed, broader-chord "Mk.XII" rudder was introduced 7-2-44, and was mandatory for a/c with the rear fuselage fuel tank (probably low-back XVIs only.)
Wheels remained 5-spoke until July/August 1944, and the 4-spoke, being stronger, were intended for aircraft carrying bombs under the wings. The so-called plain hubs were simply removeable covers, designed to combat mud, snow, or sand, which could be fitted, or removed, at will.