I've just revived my 500C/M and A12 magasin back from service. And i can't release the shutter with the magasin attached to the body.. I have tried everything i think.. Untill i discoverd one thing
What you see in the red ring are the thing the pin that shoots out the camera's back when you press the release is meant to push, and the blocking lever that prevents that pin doing so, and with it the release of the camera as long as the dark slide is inserted.
So do what Mike suggests, and pull the dark slide out.
And be sure that both the body and the back are wound. If old style the two should have a white circle in the window on the side. If you have shot 12 empty shots open the back and take out and put the reels back in to start another 12 shots.