Something simple that points to "down" seems to missing in the images. That something would cross the protractor and yield elevation.
You may notice the piece of wire that suspiciously looks like an indicator next to protractor.
The biscuit box has a hinge on the open side and the protractor has a hole at the intersection point of the radials for the angle. The mirror is fixed to some sort of wire so it pivot around the hinge. You aim at the horizon through the hole in the signal mirror and the front sight (stuck to the bottom of the box). Now you pivot the mirror to put the reflection of the sun on the front sight. The indicator shows the angle of the mirror.
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