The 1880 census says he was born in NB.
"United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MFQW-3RZ : 15 August 2017), Thos C Sprague in entry for Isaac Sprague, 1880; citing enumeration district ED 168, sheet 63C, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d), roll 0489; FHL microfilm 1,254,489.
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A birth date (Sprague-609 born 12 Sep 1871) should not be within eight months of a sibling's birth date (Sprague-610 born 9 Mar 1872) unless they are twins born within a day of each other.