Just conjecture really, but judging by the position of the 'keyhole', it might be a lock for interchangeable barrels. Swapping calibers in the field doesn't really seem practical to me, not in this type of rifle, so requiring some sort of 'key' to swap to the 9mm barrel seems like a good idea.
"If you carry a gun, people will call you paranoid. That's ridiculous. If I have a gun, what in the hell do I have to be paranoid for." - Clint Smith