Grandpa Mulford of Harvard, Illinois

I bought an antique photo on eBay that was made by a photographic studio in Harvard, Illinois. The photo shows a man perhaps in his 50s. The photo is a carte-de-visite, which style was popular from the 1860s to at least the 1880s. On the back of the photo it says: "Grandpa Mulford, Mark's Grandmother Pierce's foster father."I wonder if "foster father" is correct, or should that be step-father? It sounds like Mr. Mulford married someone with children by a previous marriage, and one of these children was a girl who married a Mr. Pierce. Does anyone know Mr.Mulford's given name?