The milled "boss" was used for the extra sling swivel and was pretty much changed to a wire loop during WW1, IIRC it was supposed to be used for the canvas action cover.

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Does the triggerguard have any makers intials? During WW1 S.S.A. (Standard Small Arms) used the wire loop, actually most of the TGs I see with the wire loop are S.S.A. assemblies mismatched during FTR's.