For those interested the Sea Kings of 825NAS in the Falklands were as follows,
XZ580/272
XV700/264
XV696/268
XV663/581
XV659/584
XV654/585
XV714/586
XV677/595
XV648/597
XV656/703
There is no record of any airframe being re-coded to 91, also no other Sea King in the Task Force is recorded as being coded 91, I am more convinced that the image shows either XV663 or XV666 that are both recorded as being coded 91 at different times in the period from June 1982 to December 1982.