Many thanks Josef and David.
You could both be right. The oral history is that she was from a town near Belgrade. Going by that, Dubno would make more sense as it is closer to Belgrade, but still not ‘near’.
I had a look at the Jedwabne records and these Kosatskys do not show up. There is an Abraham Yitzchak, but he doesn’t seem to be the same one as listed on the grave.