In this article, the New York Times provides more information about the heavily advertised appearance of Mozart’s violin and viola at the 2013 Boston Early Music Festival. There will be a concert on both the violin and viola (joined by fortepiano and clarinet), and the instruments will be on view at the exhibition on Wednesday morning.
Apparently Mozart never performed publicly on the violin, and this instrument was left behind in Salzburg when he moved to Vienna. He did like playing viola, and this instrument was owned by his widow after his death.