The change to underslung steering arms came after the change from 2 to 2a - like so many supposed distinctions between 2 and 2a! The only change I have been able to positively identify as exactly at the 2/2a change is the increase in capacity of the diesel engine.
Add to this the fact that assemblies such as complete axles are interchangeable from the first Series 2 to the last Series 3 .............(and the fact that any Series 2a is at least 38 years old, giving plenty of opportunity for this).
John

