My fathers maternal grandfather have always been "unknown" for us. The church-records don´t give any clues and since my grandmother was born in 1908, there are no one still alive to ask about it, that may have had any knowledge.
A rather vague story in our family said that the unknown father could have been a "red-haired sailor".
(Actually it was said he should have been a "captain" on a boat...)
My grandmother was extremely red-haired but neither her mother or any of her half-siblings were, so maybe it could be the truth about him being red-haired?
My grandmother gave birth to 10 children and almost everyone of them are more or less red-haired!. Also many of her grandchildren are red-haired in some degree, even myself and my son have a trace of it...
In Sweden there are of course red-haired people, but they are not common, perhaps 1-2 % of the population is said to be red-haired, some experts say it can be even less.
According to Wiki, red-hair are most frequent in Ireland and Scotland.
Today most of these matches are quite distant, but sooner or later a closer match will show up and make things more interesting.