I didn't intend to reply again on this matter, but since someone else is now having a go at me I suppose I have to. Hopefully this will be the last time. Once again, if George wishes to have the last word he is welcome to it.
George, Stroebel can say what he likes. 'M' synchronization occurs before the fully open position because bulbs took a relatively long time to build up to their maximum output, but electronic flash is much, much faster. Conventional leaf shutters are designed to trigger electronic flash at the point where the shutter blades reach the fully open position. (If you really don't believe me I suggest you dismantle a few sometime - you will find it very instructive, and provided you take care with it it is really not as difficult as many suppose.)
If you want to be really picky it is just possible that the circuit might be closed a minute fraction before, but certainly not so as the blades are still partially covering the aperture (even at its maximum) - so the shutter is, in every meaningful sense, fully open.
If this were not the case low power flash bursts (which may be effectively over within a few 1/100000ths of a second) would be spent before the shutter had fully opened. As it is though, any synchronized shutter which is working normally will be perfectly okay with the shortest of flashes even at their typical highest speeds of 1/500th.
I strongly suggest to all those that think that Stroebel has said someting else than he said to take his book and to learn about the flash synchronization from him. View camera technique, Leslie Stroebel, 5th. edition, Focal Press, p.68-72. Without reading it you can make teories about what he said or didn't say but they are totaly useless for those who read what is said and what isn't said. A lot of diagrams about the different delays and synchro problems are there too...
I have followed this thread with interest. I would like to add my comments. I own an old Alphax Synchromatic that discharges my studio flash before the shutter blades have reached the fully open position. This can be easily observed by looking into the lens while the shutter discharges the flash in a darkened room. As long as I stop this lens down to f/8, this problem is not an exposure issue, but if the lens were left opened to the widest aperture, f/5.6, it would be. I can say with confidence that with this particular shutter, the shutter blades definitely do not have to reach the fully opened position for the flash to discharge.
This old shutter does have both X and M sync settings, but there is no problem with my having used the incorrect setting. If the M setting is used, my flash fully discharges before the shutter even begins to open. This problem of discharging the flash too soon, exists at all shutter speeds including B and T.
It is very easy to see if the shutter has opened when the flash discharges, and theoretically one should also be able to determine if the shutter has closed before completion of the discharge. Just look into the lens as the shutter is tripped, and if you do not see the shutter in both the opened and then closed positions during the brief flash of light, the flash duration is definitely longer than the exposure time. Optimally, one should be able to see the shutter only in the fully opened position.