Hello
I see that your couple had at least 3 children - all born Abercorn, West Lothian
1. Betsy Boag ch. 19 Nov 1843
2. Agnes Nimmo Boag ch. 30 May 1847
3. Charles Boag ch.08 Sept 1848.
As you have the parents names - have you searched
www.scotlandspeople.gov.uk to see if they died post 1855? I'll go and have a look now.
Update; In all of Scotland - 1855 to 1920 - there is only 01 Margaret Colbert/Cuthbert/Culbert death. She is down as Margaret Culbert and dies in Abercorn, West Lothian in 1889. Her death entry should hopefully give both parents and her husbands name. In the same timeframe in all of Scotland there are 2 William Boag death. Have had to stop due to slow connection problems.
Regards, Steve