Other than bowing marks, I can't think of any symbol that is universally used.  Eyeglasses, backward arrows, and squiggly lines can all mean the same thing, but different players will use them in different contexts.  When I'm playing from a rental part that's been all over the world, I'm amazed/intrigued by the different ways folks mark 'em up.  

Abbreviations are a different matter, or should be; what ones have you encountered that you didn't know?