Hi Max,
I have a Hanhart double-pusher chronograph (wristwatch) that had this EU scale and a telemeter scale. I have been looking for the answer to what the EU scale is for and came to this topic. Your explanation is indeed of a telemeterscale but I think the scale on John's is different. It would also not make sense as the EU scale gets smaller the longer the time takes. And I would think the longer the distance between the flash and the bang would mean a bigger distance. I do not have any answers but Entfernungs-Unterschied means something like "Difference in Distance". I hope mabe we can together think up another explanation (or better, I am wrong and this already is the answer)
Regards,
Neth
