Originally the AR15 / M16 used a machined Retaining Pin to hold the firing pin in the Bolt Carrier. Eventually someone figured out this part was probably way over-engineered and just used a simple Cotter Pin instead... hence the Cotter Pin we still use today.
In the Pull Down Menu above you can choose which you need.
Note: Some shooters see the Retaining Pin as a "Performance Upgrade" since it is certainly much stronger vs the Cotter Pin.
Made in USA
This is the real deal. So glad to have found it. No cotter pins for me if I can find this version, the original.