What does this mean, please?

[Penholder]

I'm attaching a scan of part of the 1921 census with 'MK' in the column for marital status.   It looks as if it has been added by the enumerator.   It has me baffled, but there is something something similar for the next person enumerated (supposedly her lodger) in his 1919 Fishery Reserve enrolment document.   Hopefully I can attach that too but if not I'll add it next.
Many thanks,
Diana

[Penholder]

Sadly the first attachment didn't attach and I can't add it now even by changing its name.   It definitely says 'MK' with the next 3 entries saying 'single'.   The first of those is the chap whose Fishery scan has attached.

[Gadget]

Have you considered  N/K - Not known.

[Penholder]

Another attempt at attaching it!  I think it's 'MK'

[Gadget]

Can you give the ref to the household or other identifier so that I can look at the entry.

[Gadget]

I think these are the possible answers to the column that you show:

From ONS Guide to the 1921 Census

[Gadget]

From what I have discovered, the person is widowed. That column in your snip is really a double question - marital status and parents alive/dead if they are 15 and under.   I don't see what MK can mean. Also, very little information is given for that person - just name and age.   As your clip shows, the letters are in a different hand than the person who completed the form as marital status was not completed, so it could be MK, maybe missing an N - meaning marital status not known or NK - meaning  the same, It should say 'widowed' 

Gadget

[Jon_ni]

anything in green ink was added by the clerks when they were collating info for producing their statistical reports (rather than the enumerator who collected the forms). Whatever it means is a code they used for the black ink on the page entered by the householder. From the comments the person was an adult with no marital status so is some sort of code for what they entered on the person's punch card. There is a picture of the mechanical machine they used to process the data in the days before computers in this blog https://www.findmypast.co.uk/blog/family-records/taking-1921-census

and of a card https://mlfhs.uk/blog/census-1921-how-the-census-was-processed

below the Findmypast picture is the explaination "Before the census schedules reached the punch-card girls, however, each one was examined by a clerical officer who made annotations in a distinctive green ink. Presumably, green was chosen as a colour it was thought unlikely that householders would use it when filling in their forms. Principal among the green-ink annotations is the coding of occupation, but you may see others in different parts of the forms."

[Gadget]

Hi Jon - an aside:

Those machines were used well into the 1950s and 1960s    I was part of a team who designed a medical data base in the 1970s. The data on the cards  were transferred to computer and a large research DB resulted.