The trick here was the use of a very old Strobonar 282 flash, a small automatic unit that was built before thyristors. Instead it used a second flash tube called a dump tube inside the unit to cause the flash to shut off. It was capable of exposure down to 1/50000 of a second. I made a scoop of paper taped to the front of the flash to divert light straight to the eye and force it into the fastest speed, then experimentally calculated the manual exposure. The thyristor types could only go down to about 1/20000 of a second the last I bothered to check. Improvements may have been made since then, so your mileage may vary.