From FAA:
The FAA no longer issues numbers beginning with NC, NX, NR, or NL. On some older aircraft, these numbers may be displayed in accordance with FAR Part 45.22.
NC: Commercial
NX: Experimental
NR: Restricted
NL: Limited
Check here for an extensive history of US N-Numbers:
http://www.aahs-online.org/articles/N-number.htm