If you have the camera on you, open the back, set the shutter to B and fire/open the shutter so you can observe the plunger and pin from the inside. I had a problem with an Edixa where the pin was getting stuck between the plunger the bottom of the lens throat, I was able to see it happen by doing this.
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.