The hole is a gas vent.
James,I have seen the atrocities of a side mount like a Weaver "N" drilled and tapped by someone who grossly overestimated their skills and sobriety.
Yes,none of the holes in line,a few extra holes,a mix of screw sizes,up through 1/4-20,and ,of course,the brass round head stove bolt with a goobered slot.
It was nice to see this rifle was well done.
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