Apple Magic Keyboard for the 12.9" iPad Pro 3rd gen through 6th, will be most suited for the 3rd and 4th gen iPads and will work functionally with the 5th and 6th, however it may not close properly due to differences in thickness.* The silicone over the hinge behind the iPad has split from use on the left side, however the keyboard still works as intended. If you have any questions, please feel free to ask.
eBay