Uusi wasn't that far away, so we went ... the librarians were again very helpful, and there were lots of books about Finnish marine history, including several with well-known names from the Kokkola region. The Parish Office was contacted for information and someone said that there was a cross in the cemetery marked as Anton Söderman, and that they could give us a map of it... "tomorrow". As we were due to leave on a ferry back to Sweden at 9am "tomorrow" we went to the graveyard and walked it over & back... for several hours. There was no cross, although graves for a large family of Södermans, with a patriarch Gustaf.
I have since spoken with the cemetery office - it was a different surname...
There is no grave for Anton in Gamla Karleby, although it is possible that he is in an unmarked plot with his brother Wiktor, and alongside Jacob Jessberg. In books in the Uusi library I found evidence of the Jessberg family sailing the shores (see entry about Brita Caisa xx add link xxx) between Kokkola and Turku.