1823 November 14:
Dover post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
date?:
Dover post office.
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1901 February 25 through 1904 August 13:
The circular postmark is 28–30 mm in diameter.
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1922 December 1:
Dover post office became renamed as Dover-Foxcroft post office. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
Dover-Foxcroft
1922 December 1:
The former Dover post office became renamed as Dover-Foxcroft post office. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
1927 January 14 through 1944 March 29:
The circular postmark is 20–21 mm in diameter.  
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1949 November 26:
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today:
Dover-Foxcroft post office is still in operation.
East Dover
1839 October 19:
East Dover post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
1935 February 6:
The circular postmark is 32–33 mm in diameter.
(click on image to enlarge)
1955 March 31:
East Dover post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
Foxcroft
1821 June 19:
Foxcroft post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
1838 October 9:
Manuscript postmark. (notation; letter)
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1840 February 4:
Manuscript postmark. (notation; letter)
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1845 May 17:
Manuscript postmark. (notation; letter)
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1850 August 31?:
Very faint stemped circular postmark. (letter)
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1851 January 23:
Faint stemped circular postmark. (letter)
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1881 December 26:
The circular postmark is 27 mm in diameter. Postmaster was H. C. Purington.
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1905 February 7:
The circular postmark is 29–30 mm in diameter.
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191- February 21:
The circular postmark is 25–26 mm in diameter.
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1917 October 31:
Foxcroft post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
South Dover
1835 September 5:
South Dover post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)
1898 April 8: [year determined from reverse]
The circular postmark is 27–28 mm in diameter.
(click on image to enlarge)
1903 March 31:
South Dover post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide.)