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It is an interesting coincidence that that 1/4 inch diameter hole is dead center on the slot for the trigger housing spline, and the start of that slot. And, another coincidence that on the M1 there is a 1/4 inch diameter hole in the exact same location relative the the slot for the trigger housing and it's start point even though there is quite a difference in the width an breadth of the receiver leg....

This gives great credence that the hole is related to the slot for the trigger housing spline. And, if you needed to cut a lead in chamfer for that slot, how would YOU do it?

Rectangle Font Parallel Slope Schematic


I will leave it as an exercise for the reader to go over DWG 6528291 and prove it to themselves.

Rectangle Building Font Line Parallel
 

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We can be sure that the hole was not put there as just a convenient hole to hang it on while Parkerizing. No one in their right mind would add a step in manufacture for a “hanger hole”. Especially, when there are at least half a dozen other places that can perform the same function that are essential to the design (rear sight holes, selector axis hole, clip guide hole, safety bridge, etc)

There is also the issue with the tolerances.

Plus or minus 0.005” on the vertical, and plus or minus 0.007” on the horizontal. That is way too loose if the hole is there as a fixturing point, and further, if it were a reference point it would be reference by more than just two dimensions. Look at the front face of the receiver, there are dozens of dimensions off that point, same for the barrel thread PD. To further drive home the point it is NOT a fixturing point, just look at the Quality Assurance Provisions (QAP) sheet. There are 253 dimensional inspections listed on the QAP, the location of that hole is not even mentioned once.
 
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