Abstract In the Old West Frisian charters, weak personal names quite often feature the ending -en in both dative and accusative contexts. In this article, it is shown that the data do not indicate that this foreign (Middle Low German or Middle Dutch) case ending is intruding upon the original weak case ending -a in Late Old West Frisian. Therefore, the initial hypothesis is that -en might function as a kind of differential object marker, just like the ending -en with personal names in modern Mainland North Frisian. Since -en is limited to the formal, written language of the charters, however, one is drawn to the conclusion that the ending indeed serves the purpose of object marking, but purely as a notorial convention.