Looks like the opening for an index pin on the shutter. Many shutters will have such pins on the back to stop the shutter from rotating. Most such hole for pins were for specific shutters so that the cocking mechanism, aperture dial, etc. ended up in the correct orientation for either convenience or to mate with items on the camera like a cable release or flash solenoid.
I have a Kodak Ektar 105mm in a Kodak shutter with a peg that is a bit over 2mm in diameter. That's a Speed Graphic or Crown Graphixc 2x3 board shown, and Kodak lenses and shutters were common.
Still, you are right that it looks big even compared to the one I have. Maybe there is another purpose?