These were designed to stop the lens/shutter mount moving on the lens panel.
External flash synchronizers - a solenoid activated by the flash that trips the shutter - were used with Speed Graphics 1920 - 1960.
If the lens rotated even an ooch the flash synchronization would be off.
The pin kept the lens from rotating in use. It also allowed swapping lenses on the same lensboard.
Solenoid flash synchronizers are useless with electronic flash, instead the lens shutter has to trip flash. It is common for a camera to have a synch solenoid for flash bulbs and an X-synch-only terminal on the lens.
The solenoid approach was used as it worked with all existing lenses when it was introduced. As the solenoid stayed with the body when the lenses were changed it was far more economical than adding synch to every lens.
Graphics also had a synch terminal on the rear focal plane shutter. The shutter curtains were so slow the same synchronizer could be used trigger either bulb or electronic flash.
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