Another that shows the directions.
I think yours is an earlier one (or the "original" that was copied). Though it appears yours has a screw to adjust the length instead of moving the back slide plate in and out. Interesting that it's both a buttonholer and zigzagger - though if you move the fabric around, you could use just about any zigzagger to make a buttonholer. You just need a really tight zigzag.
http://www.ebay.co.uk/itm/Automatic-.../201216122366?