Quote Originally Posted by Chan Tran View Post
There is no such pin when open the back of the camera. There is a bar that push the pin. This bar is pushed by a level. This level is mounted on the side of the mirror box but it can not be seen unless disassembled.
Funny, went to check and my rewind knob was missing. Be sure to cinch that down tight or it might fall off. Retraced my steps, went back where I was walking today and found the knob.

Now for your problem. With the lens off, inspect the bar that presses the pin when the shutter is activated. S3 does not have a meter switch, been a while since I had an S3V. The bar might be different than my Spotmatic F. On mine the bar is bent back on left and right sides so you can remove the lens or attach it and it won't shear the pin as you screw the lens on.

Check the shape of the bar and make sure it does not catch on anything. Maybe it's a little bent and hits the inside of the lens mount instead of moving forward freely all the way?

Good luck solving the problem.