I have a couple of Crown Graphics that were from the late 1940's and the Kodak Flash Supermatic shutters have both "X" and "M" sync.
"She's always out making pictures, She's always out making scenes.
She's always out the window, When it comes to making Dreams.
It's all mixed up, It's all mixed up, It's all mixed up."
From It's All Mixed Up by The Cars
There were fast peak flash bulbs that used the zero delay of X-sync