What is the material of this tow bar?
Aircraft towbars are almost exclusively aircraft-grade aluminum or
  steel in construction  to meet or exceed the working load. Not only
  must the towbar be able to safely pull/push the vehicle, it must also
  be able to withstand the towed vehicle's inertia.


Source

Although there are several components that make up the tow bar they all must be strong enough to withstand the forces exerted on them. That's what the label on your picture is for, it dictates what aircraft you can use the tow bar on. Larger aircraft = Larger more robust tow bar.