I have an book "Falklands The Air War" in my collection.
It includes details on types operated by various units on both sides including registrations with codes and their deployments/fates etc.
I cannot find any HAS2 or 2A's listed with the code 91.
The only 90's coded ones listed are XV677 (CU/595) and XV648 (CU/597). They are not listed as having any code changes during the deployment.