I'm beginning to think it is just held by a tight fit(friction) with the hole. It never reaches the firing pin. The one i'm working on just falls out if tipped over to the side. Perhaps some light peeining?
The pin is held in by the ejector port cover. Item #15 on schematics, usually made of plastic. When you slide the bolt back into the receiver the port cover slides in with the bolt and it moves back and forth with it during operation of the gun.