Can't shed any light on your particular bike but it doesn't at all surprise me. At the time Francis Barnett and James were being amalgamated at a great rate and the parent group, AMC, were going out backwards even faster.
AMC were always good at the parts bin shuffle producing 'new' or 'improved' models made up from the same old bits hoping to attract a paying customer. Your bike is a classic example. It's nothing new, just the same old stuff wrapped up with a different ribbon. In the same period as your bike they were selling in the USA another James/Fanny Bee lashup as the Matchless Pinto but it was still just the same old stuff but with a Matchless badge.
It didn't make any difference, the Japanese had arrived and this type of British lightweight just couldn't compete. This was the time when I bought my first Suzuki. It didn't just run rings around its equivalent British lightweight, it tap danced, sung the Hallelujah Chorus and played the ukelele while it did it. Riders took one look at what the Japanese offered back then and British lightweights were dead and buried.
I don't mean to sound negative. We love the old dears and they have their place in British motorcycle history and they are certainly loved in their old age but "rare" implies something exotic and mysterious and exciting. What you have is a curiousity, definitely not rare and exotic.
Cheers,
