The building is in remarkable condition, including an unaltered main doorway with what appears to be original ironwork beside the low stoop:
Brooklyn Daily Eagle, October 1, 1886, p. 4 ("A Great Gain")
The date of the article matches a date at the top of the building, set into yet another glorious terra cotta panel:
In 1889, a dollar went a long way at Mr. Niemitz's store:
Brooklyn Daily Eagle, May 16, 1889, p. 4
UPDATE: How can we be sure that all this refers to the northwest corner of Union St., as opposed to some other corner? The Brooklyn Daily Eagle of March 20, 1889 carries an article about "strange doings" in a house at 158 Berkeley Place ("She Denies It", p. 6) in which the reporter quotes Henry Niemitz, "the grocer at Seventh avenue and Union street", and states that "the rear of his house overlooks the rear of 158 Berkeley place." Berkeley Place is of course north of Union Street, and 158 Berkeley Place is west of Seventh Avenue. Case closed!