While I was in Spain I got a reply from Jack at Franklin Frames to my final email asking when the frame was built:
I thanked him and asked if he knew why it had both an eccentric bottom bracket and a derailleur hanger. He replied right away that it was probably to be able to switch between gears and single speed. I’m not sure I buy that given the cable guide – I suspect that the original setup used a sturmey-archer hub.
I updated the as-built table.