The mark you have there is not identified in any of my American reference books. Some have jumped to the conclusion that it's a mark of the National Silver Company, New York City. I can't confirm this, or disprove it. I'll keep looking.
Hello Miles;
The style of that mark makes me think it could have been used from the 1920's up to the present day.
I have gone through all my American books and searched the internet, but could not positively identify the mark. Checked a couple of other American silver websites that discuss silver and no one is sure.
National Silver is the most likely candidate. I never realized how little information there is on the company. They were founded c.1900, remained in business until at least 1956.
What is the possibility that this might be a Newburyport Silver Co. 1905- 1914 mark, only in a shield instead of the retangular box shown? They used the N S Co. mark as well as the N.S.C. mark.