Can you tell how the fabric is located with respect to the foot? When I was younger, I had a sewing maching that was straight stitch only. It had a foot that looked similar the ruffler/pleater feet out there. But the foot fed the fabric and moved the fabric left and right to create a zig-zag stitch. It was the only way my sewing machine could do a zig-zag stitch.

I'm thinking that is what your foot might be.