seems in some (i guess earlier) prontor-s shutters cable release socket is directly on the shutter, other models have it in the shutter release buttons.
the bessa I for example..
Yes some don't have cable release sockets. and the same goes for Vario, Pronto, Prontor SV & Prontor SVS & Compur shutters, on folding cameras. If there's no socket on the shutter it's on the shutter release button on the camera.
Not directly related to the original question, but of some historic interest:
Apparently the Gauthier brothers have actually invented the cable release with a conical thread 1908 with their ISBO shutter (they invented the Prontor shutter in 1935).