1885 February 6:
Cousins Island post office, a summer office, opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
date?:
Cousins Island post office.
(click on image to enlarge)
1914 August 27:
The circular postmark is approximately 31 mm in diameter.
(click on image to enlarge)
1958 October 31:
Cousins Island post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
Littlejohn Island
1910 June 29:
Littlejohn Island post office, a summer office, opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
1914 July 16:
The circular postmark is approximately 31 mm in diameter.
(click on image to enlarge)
1958 October 31:
Littlejohn Island post office closed. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
North Yarmouth (1)
1792 June 12:
North Yarmouth (1) post office opened. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
1846 December 9:
  The circular postmark is approximately 30 mm in diameter. (letter [page 1], letter [page 2])
(click on image to enlarge)
1850 April 17:
  The circular postmark is approximately 30 mm in diameter. (letter [page 1], letter [page 2])
(click on image to enlarge)
year March 2:
  The circular postmark is approximately 30 mm in diameter. (letter)
(click on image to enlarge)
1852 January 22:
North Yarmouth (1) post office was renamed as Yarmouth post office. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
Yarmouth
1852 January 22:
The former North Yarmouth (1) post office was renamed as Yarmouth post office. (source: Maine Philatelic Society. 1995. The Post Offices of Maine: A Rarity Guide)
1939 December 4:
The circular postmark is 26–27 mm in diameter.
(click on image to enlarge)
1950 August 5:
The circular postmark is approximately 27 mm in diameter.
(click on image to enlarge)
today:
Yarmouth post office is still in operation.