I saw a list of what was the difference between the SUB and EPB, and although the basic body and frame were the same (a number of SUB cars were converted to EPB and remarshalled), the systems differences were substantial. Not just the brakes but everything.
I was quite surprised in using a SUB from Waterloo in what must have been 1983 to find the guard dispatching it at each station by holding a green flag, and the driver looking back. Having known dmus since they first came along, and Wirral electrics from our time there, older than SUBs and which would have been getting on for 50 years old by that time were they not long replaced, I really couldn't believe the SUBs didn't have starting bells/buzzers; they had no low voltage system.
Regarding the timber/canvas roofs, if the SR was anything like Swindon, as BR moved into all-steel vehicles it was a real issue what to do with the substantial force of bodybuilding carpenters at the main works; I believe there was a longstanding issue that those working with steel on welding/riveting were a different union (boilermakers?) to those working with timber, and something had to be found for the existing workforce to contribute; of course, the carpenters continued to work on the interiors.