The stitch width lever controls the needle position. To the far left or one mark it's in SS, then in ZZ, anything to the right of the far left setting, it causes the needle to move back and forth. The lever above it could be a lock to hold it where you set it.
You can get off set straight stitch feet for your machine at Sew-Classic. It's much easier to do straight stitch with the proper foot.
Joe