The M55 self-propelled howitzer has what looks like a toothy bulldozer blade going the wrong way, mounted at the rear of its turret. When you’re throwing 155mm shells around the battlefield, it doesn’t work to simply cruise into a parking spot, pull up the emergency brake and hope for the best. The M55 needed something to keep the machine from rolling backward after every booming shot. The result was a “recoil spade,” which could be dug into the ground to keep the M55 reasonably fixed after every shell rumbled downrange. When you’re ready to move, a small cable and winch pull the blade back up into travelling position.