It is the transmission kick down rod. When you hit the gas on the freeway, the throttle on the carburator will push back on this rod and cause it to kick down to a lower gear(passing gear) so that the car will accelerate faster instead of staying in high gear and accelerating slowly. I'm sure you have noticed that when your crusing in high gear(3rd or D) and you jump into it or floor it, that it kicks it down to a lower gear and the RPM's go up and after you get to a certain speed or let off the gas it goes back into high gear, that is the transmission kick down at work.
Here is a link to a place that has them:
The Paddock