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2.4 HISTORICAL DEVELOPMENT: THE 1879 UPU AGREEMENT AND THE «OPTIONAL PERIOD»
1879‐1886: THE «OPTIONAL PERIOD» ‐ MORE AND MORE COUNTRIES JOIN THE AGREEMENT
1883 San Salvador reply card, used back from Antwerp/Belgium in 1888. San Salvador joined 1883.
Swedish reply card used back 1888 from Wiesbaden/Germany. Sweden joined the agreement in 1883.
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