You are right re rtr models, which is a shame. However, Comet do the three window end and a couple of easy possibilities exist: grab one of the old Grafar Suburban brakes from eNay or swap meet, and just replace the end. Not exactly finescale, but passable. The second possibility is to use a Ratio brake third and do the same. I am not an LMS man, so don’t know whether this latter resembles any prototype, but it looks as if there should have been one!