Thanks all for the replies.
Leaf shutters do synch at all speeds, but some provision must be made as to when the switch will close the synch. Electronic flash will not synch at any speed if the synch is set at M or M-only. That was one of my concerns. I've some cameras with synch contacts and M-only synch which won't work with electronic X flash. For instance, my Kodak Pony 828 has a delay, so flash fires before the shutter opens. The Retina II shutter isn't marked with any X or M setting, and dates from the time of the Pony 828.
However, I've found that my Retina II has X synch. So no probs at all with X flash. Will go up to 1/500, excellent for synchro-sunlight flash.