A highway leading westward from NY 421 southwest of Warren Point to Horseshoe Lake was completed . It became part of an extended NY 421 by 1968. At the time, NY 421's western terminus was located at a railroad crossing in the isolated hamlet of Horseshoe, situated at the northwestern corner or Horseshoe Lake. The former routing of NY 421 north to Warren Point, legislatively designated as State Highway 9106B, was initially considered a spur of NY 421; it was later redesignated as NY 971D, an unsigned reference route. By 1993, the west end of NY 421 was moved about south to its current location southwest of Horseshoe Lake.