Does it look like this one? (Kind of hard to see since it's a straight on shot).
http://www.northeastsewingmachines.c...singer_99k.jpg
It looks like it might say "back tack" at the top, indicating a reverse. It looks a lot like the FW stitch length adjuster, which does also have a small nut that you can screw up or down to stop the lever. I haven't seen one in person though. Both mine have the "double pane" with the red indicator.