That will actually be an amazing prototype, Andy. Loads of interesting and very unique stuff there. I will delve into Senior's stuff to see if there's a pic of that van from one of his two visits there. I'm pretty certain there isn't, as he wasn't a prolific photographer, but no harm in looking.
Where did you get the carriage drawings, as a matter of interest?
I will try to find out for you, if you're interested, what colours the wagons would have been. Most, obviously, would be have been of C B & S C origin. Visiting GSWR ones were either all black or very dark grey at that stage. CBSCR coach livery was olive green, possibly similar to one of the shades the English Southern Railway had at one time, though nothing like CIE green. Lining and lettering on CBSCR stock was yellow.
In early GSR days, everything was a very deep maroon, with a distinct brownish tint. Within your period, chocolate and cream appeared but only for main line stock, and certainly nothing that would have gone anywhere near Courtmacsherry! "LMS"-shade maroon only appeared - again on main line only - in 1933, so that wouldn't have been about there until the late 1930s.
The CBSCR painted locos the same olive green as coaches, but I have no idea if the aforementioned locos had it. Pre-1925 "St. M" & "Argadeen" may have been black, but obviously after that all was grey. The J26's were never anything other than grey in post-MGWR times.
Just an aside - hope it's helpful!