Bjorn: I am the owner of the Mando Guitar you have pictured on your website. It can be seen on John Williams's website here:
http://www.homestead.com/aurs3/leftybass.html
I have made attempts to contact the man who had the instruments custom-ordered, Dr.Richard Ferrara, but I've been unsuccessful so far.
As far as I know, only three Rickenbacker Mando-Guitars were produced, two of them going to Dr. Ferrara and one as a 'spare', in case there were issues with the other two. My Mando-Guitar is the one mentioned in Richard Smith's book, and I am the only owner other than Dr. Ferrara.....I understand one exapmle is still owned by Dr.Ferrara, and it is Mapleglo. The whereabouts of the third 'spare' Mando-Guitar are unknown to me at present.
It is essentially a 6006 Banjoline with a different elongated headstock to accomodate 8 Kluson tuners.
Bjorn, I would have sent more pics to you, but the pics that Stan Werbin at Elderly Instruments took are so good I knew I couldn't do better!!! LOL
As far has the "sound" of this instrument, just picture in your mind an octave mandolin with a Rickenbacker 12-string sound, and you've got it.