An EJ-7 interlock is an engineering tool used to access or close panels. It is made up of Tritanium composites. With an EJ-7 interlock, one could access every critical system on Deep Space 9.
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 realised 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")