Hi Chris and Sven,
The bike in the photos looks to be a later version of the S6 (Cast iron brake drums, battery mounted at back of petrol tank, and late type 4 speed gearbox). On my 1930 model, the BTH cutout is mounted vertically on the lower tank rail so that it is concealed in the petrol tank tunnel. The mounting plate for it is only secured at the front end, with the rear end profiled to rest on the tube. (On my bike, I have attached the rear of the mounting plate to the battery carrier, as I don't like the idea of one end vibrating against the tube - this could cause the onset of all sorts of electrical problems!). The 1/4" bolt that secures it goes vertically through the socket of the front lug for the lower tank rail (depending on the model - S6 or T6 - this fixing may also include the front mounting for the tyre inflator).
Regards,
Eddie.