This may help you understand MULN wonderful shorting position senario. Putting a certificate number on a naked short is like putting a social security number on a ghost...not happening my friend. They are "speculative" shares being borrowed, then "failed to deliver".
"Naked shorting is the illegal practice of short-selling shares that have not been affirmatively determined to exist. Ordinarily, traders must borrow a stock, or determine that it can be borrowed before they sell it short. Due to various loopholes in the rules, and discrepancies between paper and electronic trading systems, naked shorting continues to happen."