Yep Def a ruffler. the lever at the back will change the number of stitches between each tuck.
The box off discs are used to change you stitches by putting them on the spindle at the top right of your machine. in the pick it already has a zig zag disc on it. I asume that the levers on the top are also involved withthe stiches.
Every thing I can find puts it down to the thread not going through the tension disk. Apparently you should always thread your machine with the presser foot in the up position. This opens the disk thus making sure it is located properly. hope this helps.