No last name for leads

No last name for leads

I understand that last name is a required field for Leads. In cases where there is no last name, what is best practice for a placeholder? 

Options we are considering (not including quote marks):
  1. "Unknown"
  2. "NA"
  3. "Pending"
  4. "Last Name"
  5. "-" (dash symbol)
In your opinion are any of these better or worse than others? Any risks or downsides to choosing any of these. Once we decide we want to commit to naming all our blank last names with that choice.