We already have a magnificent Shackleton collection on shipbucket, but I'm indulging my interest in prototypes and one-offs as an addition to Shippy2013 and Naixoterk's work.
Avro 696 Shackleton
The Lincoln ASR.III eventually morphed into a new design as the Shackleton. With the Battle of the Atlantic still very much in mind, the first 2 prototypes mounted chin turrets for anti-submarine combat, and rear defensive turrets. Both were subsequently dropped for production aircraft. The aircraft's initial designation of GR was changed to MR in service.
The first prototype was modified to become the Mk2 prototype, with dummy aerodynamic fairing fitted to nose and tail. The radar was moved from under the nose where many radomes had been shattered by birdstrikes, and a dummy retractable dustbin radar was installed behind the bomb bay.
With the cancellation of the initial Mk3 design, a simple upgrade was made to the Mk.2 to create the definitive Mk.3. The nose was bulged to incorporate a nosewheel assembly and the bomb bay shortened, tankage increased, and a revised cockpit glazing fitted. Eventually jet assist engines were added to the outer engine nacelles to allow increased weight take-off's for the Mk.3 - but with the penalty of significantly reducing the life of the airframe.
By the time that the AEW Gannets were retired, and before the eventual replacement of the Nimrod was developed the Shackleton was used as an interim replacement. As the Mk.3 aircraft did not have the service life available for continued use, the old Mk.2's were modified to become AEW.2 aircraft, and outlived their your siblings.
Avro 696 Shackleton ASR
To fulfill one of the Shackleton's initial design functions of air-sea rescue, SARO Mark.3 lifeboats were trialed on all Shackletons, including those in South Africa, except the AEW.2. No actual rescue drops were ever made, and the adoption of fully inflatable airdrop systems rendered the solid lifeboats redundant.
Avro 696 Shackleton test aircraft
The Shackleton never had the plethora of development aircraft like its Lancaster and Lincoln forebears.
Trials were made on a MAD submarine detection system, but the early generation equipment was not sufficiently effective.
The first prototype was again repurposed with additional sensors fitted for guided missile testing, and one aircraft was allocated to the Empire Test Pilot's School in a unique Royal Navy colour scheme.
Avro 713 Shackleton meteorological research aircraft
In addition to the 712 Lincoln, a proposal was made to build a Shackleton Mk.1 weather research aircraft. Like the Lincoln version it was not produced.