Tiller stay? What is this and how is it rigged.

I have a block with a knob to lock the lines leading to and from the block located on my tiller. My first thought was that this is used to lock the tiller in an upright position. My second thought is that this is used to lock the tiller/rudder position. I have a couple pictures but I don't know a public site to upload for a link this message. I really appreciate any assistance. I can send the pictures if requested. I'm a proud new owner of a '77 DS II. Owned for one week and have already been out twice. Once the thunderstorms leave my area, I hope to get out today.