I have found in two instances where a woman later re married the new surname is written in green above her name. One of these was in the 1980s! The other was in 1965 and it has 1/2 M/C written then the date 28.11.66, followed by NR no idea what that means as I looked up the marriage and it was 1965. Its a great help though.
The date is when the adjustment was done. The register was updated with new married names until the early 1990's
I have a similar finding. The delay in the dates does relate to the date of update (some say the date on the form filled in by the applicant) but I wondered if the M/C related to the marriage certificate to distinguish the change from other alterations because of name change by Deed Poll or adoption that were also noted? The final initials in my entry were LA, could these be codes for different geographical areas or NHS registration areas?