Hi Jonathan,
I do follow your thread so I can't understand how I missed you enquiry. Sorry to arrive too late.
The source of my info is Journal 33 of the GERS which contains an article on Conductor Guard workings. It states one set of steps were provided in the original 6 wheel stock in a centrally placed guards van and the halts arranged on one side of the line. When these vans were replaced by bogie stock they were built to match. My copy of the GERS journal disc is sadly 2000 miles away so I can't revisit the article just now.
Like you I spent ages poring over photos of GE country trains in the hope of finding a picture of both sides of the same coach or a view of the same line with step fitted stock captured from both sides. No luck and concluded the article was probably the best evidence.
Regarding my own snail like progress, yes I have (nearly) finished the Compo, my problem is distraction. I now have a surfeit of locomotives both running and waiting to build, loads of stock but no track to run on. That's this winter's task when I get back to sunny Cyprus next week.
Cheers,
Bob