Dano,
Thanks for your reply. It was Tweak who helped me the first time. If I am not mistaken, you helped me too.
The problem was not with the receiver. The detent pin moved (and still does) freely in the receiver. The problem was the selector lever. When I mentioned the selector's channel, I was refering to the channel/cut that connects the two holes in the selector lever. I'm sorry for the confusion. My nomenclature is not up to speed.
The detent pin was stuck in the 'fire' hole of the selector level. No matter how much I wiggled, pulled, pushed, hammered (rubber), and twisted the selector, the pin would not move. After we removed the detent pin and selector lever from the receiver, we tried inserting the detent pin in both holes, with holes facing down. From the 'safe' hole, the detent pin would fall from mere gravity--just like it would if pressed against a flat surface. From the 'fire' hole, once in, you had to pull it out. Again, this was when apart from the receiver. I wish we would have taken pictures.
I plan on buying a new selector level, a couple detent pins and springs, and other parts.