Thanks all. I've emailed S&W for possible warranty work and I'll just send it back to them. If they want to charge me, I suppose that's fine.

To clarify, the right hand side of the takedown pin did have instructions to Open and Close with the arrows. I pushed the takedown pin on the left as hard as I could to partially open the lower. Going to the right hand side of the rifle, I attempted to turn the takedown pin in the corresponding Open/arrow position, but it would not budge. I struggled with it for about 3 hours before finally taking a pair of pliers to get it to turn, so that's why I'm thinking it may have been warped to begin with.

I was lucky to have noticed the missing detent and springs. Does anyone know if S&W does any standard function and safety checks when they ship the rifle back to me? I'm afraid something else may have fallen out or gotten damaged, and I'm a bit nervous if I don't know with 100% certainty that the rifle is in good working order.