Hi Waarthog !

This is a fiducial ("crosshair"):



I strongly recommend the photography section from Jay´s website:

http://www.clavius.org

The photography section is at:

http://www.clavius.org/photoidx.html

... and here is the sub-section on crosshairs:

http://www.clavius.org/photoret.html

From http://www.clavius.org/photoret.html:

"The lunar surface cameras were fitted with a device called a reseau plate on which were etched these small black crosshairs. The plate was pressed against the film so that any image exposed on the film would contain a grid of these marks, called "fiducials"."