Some very old smokers came equipped with a high limit that used a capillary tube which is no longer available.
This can be replaced with bi-metal high limit assembly found HERE, and high limit assembly gasket found HERE.
You would install the new hi-limit assembly next to the thermocouple inside the meat compartment by cutting a whole to mount the hi-limit assembly, and running a wire from the thermostat to the hi-limit, and from the hi-limit to the burner.