On one hand Koska surname is more likely for Croatian ( or Austrian) origin, on the other hand there is only one document that states Koska and that is a report as the others on this matter, so it is also possible that is a mistake. Ok a change from Frano ( pronunciation frun-yo I suppose as there is also the common form Franjo in other languages for this Croatian name)to Frank does make it possible that also surname Koska was changed to Kosta. Maybe because of more Spanish influence in that area so that people were used to a Costa than to a Coska, but that is speculation. Perhaps you can find clues in one of Eterovich´ books mentioned in that article of my 29 Jan 2013 8:34PM posting.

Thomas