I also belong to the Cockburn Facebook group and received numerous responses to my inquiry about when Bruce first picked up the dulcimer. The consensus has been since the early '70s, but only on a few songs such as 1975's "Arrows of Light". The group admin, Daniel Keebler, who has known Bruce, since, like forever, posted this history byte...
https://www.facebook.com/photo.php?fbid=5488628841150568&set=p.5488628841150568&type=3