Hi all, thanks so much for all the replies. I've realised it's been a while since things were posted on this thread, mostly due to me getting side-tracked by Uni work. This has become something of a long term project for me, but I believe I have a few more pieces of this puzzle...
I have been able to positively identify some of the vehicles from photos, but I don't think I'm allowed to post screenshots/scans on here, so you'll have to trust me for some of the sources! The vehicles I have noted so far are:
M30219M (leading vehicle), seen here in 1965 http://www.frht.org.uk/photo-gallery-aberdeen-station.html (last image), also on page 61 of Steam in Scotland vol.2 (WJV Anderson/D Cross) on 27/8/65, both at Aberdeen Station
M30295M (different leading vehicle), very clearly seen at Perth in May 1964 on pg 16 of Gresley's A4s - The BR era in Colour (K.R. Pirt). I believe it also appears in this blurry image https://www.flickr.com/photos/aglr/16276794907/, date unknown
M30291M (likely the second vehicle), seen at Aberdeen at 2:07 of this video https://www.youtube.com/watch?v=TLjobSWQeX8 on 9/8/65. It is also likely seen in this image https://www.railscot.co.uk/img/50/249/ on 19/8/65. I contacted the photographer and he was kind enough to supply me with a higher resolution version of the image, and both he and I agree that the number is likely M30291M
All of these vehicles seem to be an LMS d.1792 POS, but differences can be seen in roof detailing and whether the vehicle was fitted with a lavatory compartment. These are best seen on pg.80 of Steam in Scotland vol.1 (Anderson/Cross), but unfortunately the photo isn't high enough resolution to make out the vehicle numbers. I believe this is related to when they were built (i.e in different LMS Periods). During this time, they still have the pick-up apparatus fitted. If anyone knows of any sources that give lot numbers/build dates/running numbers that would be greatly appreciated.
The third vehicle in the rake seems to be an LMS d.1793 (50ft POT), but I have not had much more luck in identifying specific vehicles beyond this.
There have been suggestions on other threads such as https://www.rmweb.co.uk/community/index.php?/topic/132671-lms-tpo-kits-in-4mm/&tab=comments#comment-3101654 about the identity of the 4th vehicle. There seems to be two different vehicles, one GW and one SR designed, exchanged at some point. Further research will be required here.
I will continue to update this thread when I find out more information.