An EJ-7 interlock was an engineering tool used to access or close panels. It was made up of tritanium composites. With an EJ-7 interlock, one could access every critical system on Deep Space 9. The tool was stored in Miles O'Brien's toolbox or in the tool locker in section five on level three.
When Miles O'Brien was assisting Neela in repairing a system in Odo's security office, O'Brien remarked that an EJ-7 interlock would be required to close the panel they were working on. While looking for the interlock in his kit, he realized that it had gone missing. After a search, they found it in an access tunnel, melted into slag due to exposure to plasma. (DS9: "In the Hands of the Prophets")