Colt 3x20 Japan scope

[Macon Due]

Hi,I recently purchased a Colt [ Hakko] 3x20 carry handle mount scope and am wondering what the large slot head screw on the side opposite the windage knob is for? I printed a copy of Colt's instruction pamphlent but it does not mention it. I had the same type scope many years ago but either don't remember or never knew then either. Thanks for any help.
Macon

[Dravur]

I have that scope as well. I never knew what that screwhead is for either. Tagging to find out.

[lowprone]

In all likely hood it is where they nitrogen purged the scope