1879 January 21:
South Union post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
1912 April 26:
The circular postmark is 31–33 mm in diameter. (image side of card)
1915 September 9:
The circular postmark is 31–33 mm in diameter. The outside vertical height of the four horizontal bars to the right of the circle is 19–20 mm..
1949 June 30 through 1950 July 26:
The circular postmark is 32–33 mm in diameter. The outside vertical height of the four bars to the right of the circle is 19 mm.
1953 April 30:
South Union post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
Union
1810 May 3:
Union post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
1928 December 22:
The circular postmark is 28–29 mm in diameter.
1947 January 30 through 1950 March 25:
The circular postmark is approximately 29 mm in diameter.
