Are you talking about the shaft in front, so you can hand start the engine? The shaft should have a pin on the inside where it catches on the front engine pulley. It also should have a pin on the outside (front of the tractor), where the hand crank will engage it. If you missing the outside (front) pin, a bolt will work fine. For safety reasons the inside one should be replace with a proper pin if it is missing. These pins are only necessary IF you intend on hand starting the tractor.
Reading between the lines a little bit, you should do some searching of the archive on how to properly hand start a tractor. The Allis tractors properly timed tend to be a bit more forgiving than the Farmall counter parts, but both can cause serious injury if an improper technique is used. If this tractor is a fairly new acquisition you might be better off pull starting it until you are sure it is timed properly and running correctly.