Would it have made more sense if they have intended to use something similar to the short lived Lancaster Mk VIs, the two-stage supercharger Merlins of which gave a reputed 313 mph top speed and 28,000' service ceiling for a 65,000 lb TOW? There are various versions of what they were used for beyond a Pathfinder/Master Bomber role and they were also used for development/trials after withdrawal from the front line. "...surviving aircraft were used by Rolls-Royce, the Royal Aircraft Establishment and the Bomb Ballistics Unit (BBU) for various testing and experimental duties." The history of the B VI seems very confused with FF dates in both '43 and ''44 quoted, different numbers produced/converted, and higher top speeds, JB713 is believed to have out run an intercepting Ju88 returning from a raid on Berlin. So could the Black Lancs have been additional B VIs (or an unknown different B III conversion) lost in the paper work?