I've seen it claimed, both times in polemical works claiming that the non-Chalcedonian churches are Monophysite, that people of said churches cross themselves with only one finger to signify the one nature of God the Word incarnate. However, I'm pretty sure they cross themselves with three fingers like Chalcedonians do, only left-to-right rather than right-to-left. Was there ever a time where they crossed themselves with one finger?
Now that I think about it, the three-finger cross is a new thing for Chalcedonians as well, since the old way was with two. But that sounds like a post-Chalcedonian development, since the two fingers signify the two natures of Christ. So how did Christians cross themselves before Chalcedon, and how did non-Chalcedonians cross themselves after?
How Oriental Orthodox cross themselves varies from tradition to tradition. In Ethiopia/Eritrea you are generally supposed to make a cross with your fingers - I've most often seen people curve their last three fingers with the pointer finger and thumb relatively straight up, with the pointer crossing the curved last fingers to make a cross. (There's a ton of variation though.) I don't know about the other traditions - I've only rarely had the opportunity to visit their churches, and I was more taken with the services themselves than with how people were making their crosses.