"1919 = The Convention for the Regulation of Air Navigation, as part of the October 1919 Peace Conference, created the system of international identification still in use that sets the first letter(s) as country of origin: N for United States, D for Germany, G for Great Britain, SE for Sweden, etc. This system was in use for seven years before it was formally ratified by our government.
1926 = In May the first real attempt at organization came with the federal Air Commerce Act that went into effect in January 1927. In this system a class letter C, S, or P was to be added, denoting Commercial, State, or Private. C specified approved (airworthy) airplanes used in commerce and the air mail, but this was amended in 1930 to include any aircraft meeting minimum government airworthiness requirements regardless of its use. S was for state- or federal-owned planes, with most all states requiring aircraft operated within their boundaries to bear an NC number (Oregon, where much flying activity took place, was a notable exception), but this was dropped in 1937. P only lasted until March 1927 to sort out private aircraft from C and S (no example of an NP designation was located). A limit of five numbers seemed adequate at the time for present and future aircraft, but these were all taken by 1929!
1929 = A new plan was to approve three numerals, and a suffix: E, H, K, M, N, V, W or Y. Not surprisingly, these new blocks were used up by the end of 1934.
Class prefixes R and X for Restricted and Experimental aircraft were established. A class prefix of G identified Gliders until it was canceled in 1937, with sailplanes and gliders placed the same bag as powered aircraft. The use of the letter N was optional at this time for aircraft flown within the nation's boundaries."