71st Infantry was organic to 44th Infantry Division. Company D was the heavy weapons company of the regiment's 1st Battalion, meaning your Gramps was probably a crew-served weapons (most likely machineguns or mortars) squad- or section-leader. See Message #6160.
Also, found a Mike (died 1993 in Florida) and Anna (died 2000 in Warren, Ohio) in the Social Security Index. Your Grandparents?