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The hole in the receiver and the hole in the bolt were not drilled at the same time with the same drill. I think that the hole in the receiver is larger so as to reduce the requirement for finicky alignment of holes in two separate parts.
Further, the real "Hatcher Hole" was drilled long after manufacture to give you a chance if you blew a casehead in a brittle low number action.
My Mk I has the vent hole in the bolt, but not in the receiver. The original thinking seems to have been that the bolt lug raceway and magazine well were enough.
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