The National Archives has a search facility for RG letter codes but I can't find that one so far. I might have put in the wrong letters/numbers. Have you got a clearer image or maybe someone might be able to read it.
Add - if it's CS/xxx , it could be Change of Surname
Thanks, the image is the clearest one I have
I read the code as CS/3748 (change of surname 3 July 1948???) but it could also be C3 or C5. The first change to Wilkinson I would have thought would normally be a marriage but so far I have not found a marriage in England and Wales or Scotland.
Bessie was born 10 July 1913 and died 31 July 2004. She was the daughter of Alick Bates McDonald and Lilian Deacon. She had a sister named Margaret, born 1912 and died 1944.
Bessie's estate was administered by Elizabeth Anderson Haughey (previously Emerton and before that McDonald) but I don't know the relationship between Bessie and Elizabeth and I have not found a birth record for Elizabeth so perhaps one of Elizabeth's marriage certificates might give a clue as to her age and possibly her father.
The closed entry on the register would also possibly help of course but I don't know who that could be or if that person is connected to the family - I would guess at a child of Bessie or her sister Margaret but it is only a guess.