Very interesting I would imagine it came about as a proposal when they were replacing the 4w TPO's in late 1960's they were eventually replaced with three GSR built coaches sating from 1935 which in turn became 2979,2980 and 2981 it would have been a waste of some fairly new Cravens. The three GSR TPO's built in 1935 converted in 1967/68 soldiered on in service until the early 1990's.