IIRC (it's been a while since I read 'Slide Rule') the Airspeed Ferry was designed specifically for Cobham by Airspeed. So not really an oddball aircraft but one that was handbuilt to do that specific job. It may have been a bit odd looking and the production run was short, but I'm guessing they did their job pretty well.
And a quick Google produced this:
Airspeed Ferry - Wikipedia, the free encyclopedia