What does this screw do?

The Dura Ace seat post is a late 80s/early 90s model.  My guess based on the other components; it came with a touring that had a seven speed MTB 135 OLD rear hub.  My question is straightforward: What does that screw do?  

I image that it would affect or limit the tilt of the saddle.  But asking before I unscrew/loosen it seemed like a far better idea.