It is the sight for the 203 grenade launcher that fits on the M-16 rifle to make a complete rifle / launcher out of just the rifle with the launcher mounted underneath. I have had one for years, but have no use for it other than looks.
Yep, its a quadrant site for the M203. I'm an instructor on them at work and see them regularly. It mounts on the left side of the carry handle. The known distance to target is adjusted via the "slide rule".
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