Cat B MI & Cat B MR?

I’m going through a lot of aircraft record cards at the minute and am getting a lot of ‘Cat B MI’ and ‘Cat B MR’ entries. I understand what Cat B is, but can anyone advise on the MI & MR bits?

My assumption would be that these suffixes denote some kind of modification action, rather than damage resulting from an accident/incident. I’ve come across similar treatment for RAF Sabres, which at the end of their service lives were placed Cat.4 (Rehab) for overhaul.

Any help much appreciated!