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I have never seen any like that before.

These are just my thoughts -

I don't think it's a USGI manufacture. The corners on USGI M14 extractors are pretty much sharper cut, the case head rim slot cutout behind the claw is also rough looking on this one, on all of my NOS USGI extractors the cut is sharp and very smooth, the plunger detent also seems rough and on most, but not all of the USGI pieces I have you can see some machining marks in it and the cut is sharper and surface inside smoother. Some of the contours also do not look like USGI pieces I have, just minutely different in a couple areas.
The only "O" marked M14 part I know of is on the bottom side of bolt stops, and those are H & Rs and it looks a little different. I don't believe it is USGI, SAI, or Fulton, that could be a help to it's identity.
m14brian
 

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The only "O" marked M14 part I know of is on the bottom side of bolt stops, and those are H & Rs and it looks a little different. I don't believe it is USGI, SAI, or Fulton, that could be a help to it's identity.
m14brian
USGI bolt stops are cast, so the circle might be from a mold or tree. That would also be a possibility with this extractor.
 

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It’s not an “O” or a “0”, it looks like it’s was clamped in place by a screw. It was not stamped.

Ren
 

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USGI bolt stops are cast, so the circle might be from a mold or tree. That would also be a possibility with this extractor.
Casting gates are usually broad and flat, or machined completely off so you can see them.

And, that would be a bad place to put a gate.
 

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Investment cast wax molds and MIM cavities normally have ejector pins.

Those pins show up as circular depressions of all sizes dependent on the part size.

The classic part for that here is the M14 bolt stop, that circle is the tell tale between

rolled bar stock and cast.

If anybody wants an explanation on "rolled bar stock" I will post it.
 

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Investment cast wax molds and MIM cavities normally have ejector pins.

Those pins show up as circular depressions of all sizes dependent on the part size.

The classic part for that here is the M14 bolt stop, that circle is the tell tale between

rolled bar stock and cast.

If anybody wants an explanation on "rolled bar stock" I will post it.
I don't doubt that it is cast, I just don't think that mark is an ejector pin.

They usually look more like this:
 

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This photograph is entitled:

SA-STD-120. Visual Standards for Surface Defects. In Precision Castings - Figure 3

Second row, left, shows a circle on the bottom of a bolt stop, as mentioned by M14Brian.
 

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I don't doubt that it is cast, I just don't think that mark is an ejector pin.

They usually look more like this:
I was not referring to the extractor but cast parts in general.

I gave my opinion on the circle in a earlier post.

Just to let you know, I have made plenty of molds for invest cast parts and even more for powder parts.

None for MIM.
 
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