The year, if it is the year, is only partly legible. What should I put for birth year?
Since you do have a marriage date for John Aitken, M39B-HKR the birth year could be left blank. You could also estimate the birth year from the marriage date using the term about. Looking for additional documentation would also be beneficial to you.
If you are talking about the two digit number at the top of the page, that is the page number not the birth year. You have indexed it correctly by showing the birth year as blank.
@nanaca you found the record for John Aitken using the Batch Code as an ID. M39B-HKR is a batch code for indexing and the image is a register of births. So as not to confuse indexers, we NEVER calculate or estimate a birth year when indexing, or look for additional documentation.