Getting the gun camera and gun sight to point in exactly the right direction was important on a WWII fighter. Here’s how they did it on the P-51 Mustang.
The plane’s propeller spinner and the nose of the airplane are stamped with red index lines. A mechanic rotated the prop until the proper two lines joined (either for the gun sight or gun camera).
On the back of one of the propeller blades, a target (called an “index mark”) was stamped, one for the camera and one for the sight.
Looking through the gun sight or camera (using a tool called a “line level indicator” and “camera aligning indicator” respectively), the mechanics could aim the sight or camera exactly at the index mark on the back of the prop, assuring a proper alignment every time.