We had the pin (as well as the other 2 parts) fall out of our 642 a couple of years ago, which happened within the first 100 rounds.

The only thing I was unable to find was the spring. The gunsmith at the shop where I bought the gun had one and fixed me up while I waited......

Edited to add: For what it's worth, we were only fireing standard pressure factory .38 spl loads at the time. No +P