In pictures of carriage roofs most seem to have only one pipe and some seem to have the pipes covered.
refer str394c Warwickshire Railways the coaches on the left hand side if the picture look like they have 2 pipes and I assume are 30ft 1" coaches.
2877 these coaches have only one pipe.
The drawings for the 42ft coaches in the latest publication seems to indicate only one pipe.
Was there a change in the type of gas lamp used and how was the lighting of the lamps carried out. By the passengers which I doubt or the guard.
Coach recognition is not my strong point