National Register of Historic Places. It was originally built of limestone, and covered with mastic, but the exterior has since been resurfaced and painted. (Article 27 of Helen Hansen’s book At Home in Early Sandusky provides excellent historical information about the McKenster-Groff house.)
A 1978 history of Smith, Hinchman, & Gryllis, by Thomas J. Holleman and James P. Gallagher, is available for interlibrary loan through the ClevNet consortium. Ellie Damm’s book Treasure by the Bay provides an excellent architectural history of many of the homes and businesses of Sandusky. See the City of Sandusky’s page on Historical Downtown Sandusky for an illustrated view of Sandusky’s downtown.