Well, I'm betting Jerry turned Bobby on to F50's, he'd been acquainted with 'em since about '63 already:
Eventually Bobby ordered up a couple with exotic headstocks, and I believe they were part of an ultra-rare run of about 6 with actual arched rosewood backs as well.
They've been reported here before:
http://www.letstalkguild.com/ltg/archive/index.php/t-185399.html
But the report about there being 6 of 'em ("IIRC") was reported by another member who had one of 'em, in a different thread.
And I think Danko might actually be borrowing Jerry's F50, because otherwise I've only ever seen him with a sunburst one, and I think it came later: