Wow! That cover is an oldie. Quite a unique "P.O. Business, Free" marking, too.
As you may know, Carver's Harbor, Maine ceased to exist under that name by 1879. Check this out, courtesy of the Vinalhaven (ME) Historical Society:
Quote:
According to a petition to King George III, dated 1772, Thaddeus Carver arrived from Marshfield, MA in 1762 and by 1775 he was operating a saw mill on 700 acres purchased from Francis Cogswell on the southern shore of the South Island in the area which later became known as Carver's Harbor.
Vinalhaven or Vinal Haven? Originally one word in 1789. The U.S. Postal Service changed the Post Office's name from South Vinalhaven to Carver's Harbor (1850), to Vinal Haven on October 29, 1879 and back to Vinalhaven on March 15, 1925.
According to Jim Forte's Postal History Website, the name Carver's Harbor was in use from 1850-1860 (Waldo Co.) and 1860-1879 (Knox Co.).
The Postmaster at Carver's Harbor was David Vinal, as per this record when the post office was established in 1850:

In fact, David Vinal continued as Postmaster at Carver's Harbor through the 1870s as documented here:

As for Belfast, Maine, the Postmaster at the time was Samuel G. Thurlow:
