Hi, I know that they definitely came from Rathlan Island because they are my ancestors.
Always better to get as much info locally. Have you tried calling the Burnet Building where the registry office is now situated. You might have to go through the Council number in Lochgilphead to get put through. 01546 605522 (I think)
One of the relations might be able to tell you more. If so I'll give you a contact for her if she agrees.
Burnet Building
St John Street
PA28 6BJ Campbeltown
United Kingdom