I have recently come across your blog & am very impressed with your work. & would be interested in purchasing some of your wagons, vans especially.
Regarding the left handed brake lever,this was chosen for cheapness as it didnt require extra cost such as clutch in the Morton system or a lift link for a second right handed lever, which on an individual wagon might not seem a lot, but when you are talking in terms of 10s of thousands, this soon adds up. However in 1904 the BoT changed the rules that all the brake handles had to be at the right hand end of the wagon, but , from memory,the companies with 20,000+ wagons were given I believe 10 years to convert them & then another 10 years because of World War 1., so in many cases the near & life expired left handed wagons were simply scrapped rather than being converted.So I imagine, there were probably still a number of them still running in the early grouping years,but most likely in pre-group livery. I hope that helps