The beans in my picture came from the daughter of a long time member of the Brighstone Horticultural Society, at least 30 years in the group. So if he is wrong there will be a lot of annoyed people in Brighstone who also grow the same. The first reference you gave was from our local rag where the 'Gardening expert' cannot even get off his backside to use a proper photo and has used a stock library picture of beans.
The American reference certainly has some of the purple colouring and could have easily be some crossed pollinated beans, perhaps the collector picked them by mistake if he was on a seed collecting trip. The old ones we grow are nowhere as big as they seem to be. I would expect an old variety to be small like mine as they are originally a few hundred years old, before the trend for bigger crops