If I'm not mistaken, the "firing pin stop" has been a part of the Ruger .22 auto since the days when they were being produced in a small shed. It was a part of Bill's original design, as far as I know.
That was my thinking as well. I know that on the Mark 1s at least they can fall out easily and could get missed and the bolt reinstalled which would cause a problem but I never heard of one that was built without it. Of course I am no expert either.