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Frank, CCNL 32-5 (dtd June 1979) may imply that the NI marking was for IP of S'G' but CCNL 346 (dtd Mar 2008) says the marking is for Inland, and actually lists two Inland serial #'s that the NI has...

I think that's it. See CCNL 346-14, which talks about the NI front sight also refers to CCNL 343-2 which has pictures of the NI sight just like Franks.

Frank, Is there a number on the left side??