Andrew’s death cert:
There’s a death for a Margaret McCosker regd in Londonderry on 5th June 1968 aged 66. Perhaps that’s his widow? You can view the original certificate on-line on the GRONI website, using the “search registrations” option:
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
Sorry I can’t help with a photo of the grave as I live too far away but perhaps someone in the area will see your post and be able to assist.