The major difference, as you said, is the rear. Other differences include the 'Bus Stopping' sign inside Mk2A, as well as Mk2A using taller number blinds in front (not always, as AG2906 uses taller number blind despite being a Mk2).
That's just because you never came across those poor ones. The poor ones are just rattling throughout the journey and make loud noise when braking. I've even managed to find one shaking like (and sound like) a Dennis Jubilant.
New Street Station
Yardley Wood garage
YW2453
YW2596
AG2849
By the way, there is another difference between the two types
When new the Mk2A have via blinds on the side next to the route number, but these were later discontinued with the glass reduced to show the route number only.
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