The barrel has interrupted threads which makes it an earlier one. Earlier than when I don't know. I know we've talked about when the change was but can't find it and can't remember it. I do know that an interrupted thread barrel will work on a newer receiver and that the forearm metal will also interchange. You'd have to slightly shorten the forearm tang to use the later metal though and thus lose any serial number that might have been stamped on the end. Curiouser and curiouser.

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