JW411
A particularly clever trick indeed since the Argosy wasn't equipped with reversing propellers!
Actually, all of the Dart series engine/propellor combinations were equipped with Reverse Thrust. It was only the civil variants which were "reverse disabled". It was considered that the pitch change from Flight Fine Pitch (FFP) to Ground Fine Pitch (GFP) was entirely sufficient for the braking purposes of civil aircraft. Hence, the military version was, indeed, equipped for Reverse Thrust capability (short field operations).
My source of information is directly from the RAF Pilots Notes and a copy of the Dowty Rotol Handbook; also, my basic Air Eng training also described the propellor operating system in great detail (Reverse Thrust included).
The same engine (actually a Dart 201) is fitted to the Andover C1 and, to this day, the "Boscombe" Andover demonstrates its "reverse capability" to those who wish to use it.
TCF