Nice collection of photo's Graham!
A few notes as a starter for ten...
It's an original Mk2 (later 2'z') - apart from the number, the usual giveaway if you can see them, are the gangway doors - for Mk2's they were the conventional wood frame/steel panelled door like on the Mk1's - hinged for the toilet end and sliding for the non-toilet. From the 2a's onwards (thru a-f) they were GRP bi-folding doors. Also, only on the Mk2s did they create the 48-seat variant (2+1 seating) as well as the more conventional 64-seat (2+2 seating). for the end vestibules, the Mk2's used a dark green pigment for the GRP panels and on the Mk2a's (including the bi-folding doors, the much lighter leaf green pigment)...
As far as braking, the Mk2's were built with vacuum brake only (the E&G air/disc braked vehicles were an early 70s conversion and the only exception) From the Mk2a's onwards they were all air braked only. During the late 70s and 80's a small number of the Mk2a's were altered to vacuum brake only for the Western & Scottish regions...
Leaving aside the FK's etc. as far as the SO/TSO's were concerned, the Mk2's and Mk2a's were all built with Dual Heating capability.
Apart from that there was little obvious difference between the Mk2's and 2a's - on the inside, the toilets were changed from conventional formica panelling to moulded GRP panels otherwise they were almost identical.
Other points of note, are the missing covers over the toilet water filler pipes - these got in way of the staff filling the tanks and would mysteriously "fall off" in the carriage sidings and rarely be seen for long outside of the main works! On the interior, about the only non-original thing is the seat trim - the 2's and 2a's having the usual blue/green check... and blue arm and headrests, and that late Mk2 table. The interior wood panelling just through age and repeated striping & revarnishing is much darker than it was originally, and some if it appears to have been replaced, the original (with the grain always running top to bottom of the vehicle) was a much warmer and lighter teak (well it was teak faced plywood panelling)...
I'm not sure I see anything out of place or extrta on the headstock Graham - apart from the niphan plug hanging under the left buffer which I assume in preservation has been added for connection to a shore supply.
P.s. I'm sure no one will make the mistake of painting a Mk2 SO in BR Maroon - all (unlike some of the FK's were painted as built in blue &grey only) Preservation societies of course can do whatever they like!