Would this have been part of the rumoured plan to run Rayners-Acton as a shuttle? It is interesting to note that even twenty years ago it was recognised that the Pic had a deficient number of trains to run a full service reliably over its entirety. Indeed, the 1952 tube stock proposal would have entailed ~100 7car trains albeit for a 40tph service through the centre.
To state the obvious from the numbers, there would be 20 'middle' cabs, and 16 spare trailers, so presumably some butchering would have taken place. Your suggestion of changing the car body above solebar does seem far more likely than reconfiguring space below.
Perhaps Underground News wrote something on the topic contemporaneously?