Hi Heather,
I have done some Googling on the subject and came across this piece in the GWS Didcot website:
”As an indication of the popularity of smoking 100 years ago, on 4 October 1920, the GWR issued a circular to all stations laying down that all trains had to be formed with 50% smoking accommodation. From 6 October 1930, the GWR decided that special compartments would no longer be labelled as being reserved for smokers and that smoking would be allowed in all carriages except those designated by a new type of triangular ‘No Smoking’ label. This had the effect of increasing the proportion of smoking accommodation and the GWR then started to label one in four compartments in nearly 10,000 vehicles as ‘No Smoking’.
Smoking on trains may no longer be legal nor desired these days, but even so, the exploitation of past smoking norms, to further GWR's publicity is at the very least, a remarkable demonstration of their drive and confidence to influence their customers in any way possible!”
I don’t know if that helps at all, but I guess there would have been some sort of transition period.
Nigel