Thanks for the prototype shot. I see enough from that to understand I need to do a lot more studying. These vehicles aren't covered by my sole coaching stock book: the relevant Historic Carriage Drawings volume.
Good fun isn't it ? Every time I think I've sussed it another quirk or variation drops out of the woodwork. Volume 2 of Essery & Jenkinson's "Illustrated History of LMS Standard Coaching Stock" covers general service gangwayed vehicles and can usually be found for less than the silly money that Volume 1 (NPCCS etc) seems to command. (still looking ! ).
Before I put an order in to Comet to take care of the Vic Berry pile of Airfix coaches in the loft I need to sit down with the picture books and identify some of the Port Road sets down to diagram numbers etc. The rule seems to have been 'no two vehicles of exactly the same style in the same rake' or so it seems at times ! I have a similar dillema with the Hornby coaches too, I'm undecided what to do with my three. (Troop train ? )