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That is indeed what it appears to be . I would now guess the "10" spot is really a zero and the "20" next to the knob is missing its "2" because of the housing's slope .
Chris

No , I think it may pivot left / right via the hole up front and a screw on the weapon . Could be mounted solid and the front may do windage . I think it mounts with the knob down and these are...

Ummm …. it may be a casting mark , but I see a zero , 4 lines , a long line with a 10 split on it , 4 more lines , then a long one with a split 20 .
Chris

I'm guessing it is some type of rear sight . It looks to mount on the side of the weapon in the orientation shown in the last picture . I would guess it is non-USA and is for something like a grenade...