Henry Nettleton "HN" a civilian inspector in post war years . In 1873 he inspected repair of old arms which included Spencers & Sharps. So it is possible he might be the HN , but not on the original 1860's production of the rifle
Welcome from Missouri. Looks like a nice rifle. My cousin acquired one very similar a few years ago, and his had an inlet for a missing sling swivel just like yours. He was able to buy a replacement swivel and screws for it.