A good source of Pullman information is Terry Bye's "Coupé News". I don't know if it has covered the MR cars, but may be worth a search. You can find it at http://www.semgonline.com/coach/coupe/index.html. The only picture I've seen of an MR car is in Cuthbert Hamilton Ellis's "The Midland Railway", Ian Allan, 1955, p167. The picture shows a conversion to a push-pull carriage, which would account for one of them. I am sure it is long out of print, so it is probably OK to scan it and send it for private use if that would be helpful.