My gran did all her sewing on an old Singer that didn't have zigzag....when she passed away my aunts gave me 30 of her homemade dresses to turn into quilts and I when I started taking them apart I was really surprised to see how she did zigzags.....she just zigged and zagged the fabric herself! It's too irregular to be done with a zigzag attachment, I think that she must have just sewed an angle, stopped and turned the fabric, and sewed the next, and so on. Nice sharp points so I'm pretty sure she stopped at each point; I'd think they'd be blunted if she hadn't stopped at each point.
Seems pretty labor-intensive but it worked! Some of those dresses were worn thin in spots but all her sewing held up marvelously.