As far as I recall, the only locks mentioned during the Dressrosa Arc are:
- The lock to the SMILE factory
- The lock of Law's handcuffs
Though, in chapter 747 Doflamingo destroys the key to the factory.
And in chapter 750, we can see Viola holding the key to Law's handcuffs.
So what is the key, Doflamingo is holding in chapter 752, for?