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At least one prison in Maine counted both white and Black prisoners among its population during the 1940-1950 time period.
In the 1950 census of Knox County, Maine, enumeration district 7-36 includes the Maine State Prison in Thomaston.
Page 3 of this ED lists, mixed among the "W" (white) prisoners, the following two people:
In the 1940 census of Knox County, Maine, ED 7-38 includes the Maine State Prison in Thomaston.
Page 2 of this ED lists, mixed among the white prisoners, the following person, who may be the same as one of those in the 1950 census:
There are other Black inmates on other pages of both censuses. There are also Black inmates listed in the 1930 census of Maine State Prison.
The Wikipedia page for Maine State Prison claims, without references, that in 1927 the prison held three Black prisoners in addition to 197 white prisoners.
Note that Maine is and was a very white state. In 2023 it is about 95% white. In the 1950 census, 1,221 out of Maine's 913,774 residents, or about 0.1%, identified as Black.