To add to that:
AJ Jackson, Avro Aircraft Since 1908, p.418:
"[Phase 3] had MR 3 navigation and offensive equipment together with a Mk 3 exhaust system....and were known unofficially as the Shackleton MR 2C"
Chris Ashworth, Avro's Maritime Heavyweight: The Shackleton is (as I'm sure you know, JN) good on this. It also makes reference to a planned MR 2A on pp34-36.