I have a Nikkormat where the shutter release collar and button are missing from the top plate, like that but I don't have the missing parts. It was probably knocked out somehow, although it would take a lot of force. The Nikkormat still works if you use a tool to press the shutter linkage below the button. On that F2, you would still be able to fire the shutter with the self timer, as the self-timer is triggered by the little button behind it rather than by the shutter release.