Every country is allotted a prefix which is applied to aircraft registration marks in order to identify the country of registration.
These prefixes have their origin in the conferences held by the International Telecommunications Union.
The first convention was held in Berlin in 1903 and was attended by 8 countries. No agreements were reached.
27 delegates attended the second convention in Berlin in 1906, but it was at the third conference in London in 1912 that the first codes were adopted.
The resulting 1919 convention also established the International Commission for Air Navigation (ICAN, or CINA in French).
ICAN/CINA adopted a set of marks to be worn by aircraft at its initial meeting.
In 1922 they suggested marks to be worn by non-signatory nations.
Subsequent meetings of ICAN/CINA allocated further marks in the period to 1927.
The 1927 International Radio Telegraph Conference in Washington made significant changes to the list of marks resulting in the following table.
Subsequent pre-WWII meetings made further changes.
ICAN/CINA remained until 1945 when a Provisional International Civil Aviation Organisation (PICAO) was established, lasting until 1947 when the current ICAO commenced operations.
The prefixes in use during the period from 1919 to 1939 are indexed in the pages referenced below.
The links will take you to the pre-war registers for these countries.