In 1964, the United Arab Republic (only Egypt, as Syria left the union is 1961), issues a series of 13 stamps with the flags. Each stamps featured the flag of one of the 13 Arab countries then members of the Arab league. Below is the Algeria flag stamp.
The stamps with flags were issued on the occasion of the Fourth Arab Summit, that gathered in Alexandria, Egypt on September 4, 1964.
One of the most important decisions of this summit was to welcome the establishment of the Palestine Liberation Organization, created by the first Palestinian National Council, held in Jerusalem in May 1964.
The stamp above bears the flag of the Republic of Algeria. Algeria joined the Arab League on August 16, 1962, less than two months after its independence.
Yvert & Tellier: United Arab Republic YT-611