Ive found sten Dahl  The CircumBaltic Languages Typology and Contact vol  I dont have enough access to always be able distinguish when Curonian is referring to Old Curonian ws modern Curonian dialects of Lithuanian but at least one author in that collection says that it is now generally accepted that it was an Eastern Baltic language intermediate between Lithuanian and Latvian though also mentioning some would say between E  W Baltic and discusses the morphological evidence most of which I cant see