Having re-read what there is to read about it, the guitar in F makes a little more sense to me now - as far as I understand, it was tuned A to A, so a guitar but a fourth up. Like my Yamaha Guitalele.
Makes sense like that, and I can see how apart from Barnes' specific use for it, it could be a handy studio tool.
Hans, are the guitars in F mini Barnes models including a carved top, or did they use M-75 tops for these? Carved top would be very fancy for a little guitar like that.
Seeing the photo of George Benson in Hans' book where he's playing a guitar in F, I wouldn't be surprized if the Guild "F" guitar was a big inspiration for the small Ibanez George Benson guitar.