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First co-witness compatability, w/ the M68 and the #39A2 your set.
You'll notice the second attachment point, making certain the mount does not shift or move.
You'll also notice the tiny "brush-guard" feature on the left end, above, while it may not seem like much extra for the price, try "beating feet" thru the heavy woods or jungle sometime and see how your $30-50 dollar mount fares......
Chances are, your pace thru the vines and brush will suffer considerably and while it may not mean much now, in a SHTF situation ya don't want that "cheapie" mount to become a brush and vine magnet. Of course if this is of no intrest the buy the cheap one, and hope it's the correct height.
Mike