What is this front screw?

Here's the screw: 






It also came with a Nelson slide that has a cut out for that screw:

Any ideas?

Someone wanted a reference in the scope to find the red dot faster??

Sa-tevp wrote:Someone wanted a reference in the scope to find the red dot faster??

That was my guess as well - it sits right below the dot when it’s zeroed at 25 yard and it seems to help tracking the dot through recoil