Exmark MSHN21B Used 21" Metro Walk-Behind Mower 5.5HP Honda. The Exmark commercial walk-behinds get the work done faster with an unmatched quality of cut and time-tested durability. The handles and control system are designed to make operation easier, more efficient, and less fatiguing. Like all Exmark commercial mowers, our walk-behinds are the ultimate tool for landscape professionals, commercial lawn keepers, and everyone in between.