The one with the test socket inside should be the master, but if there are no wires connected to its faceplate yet you have a working extension, then the extension must be wired incorrectly. I guess that you have an old-style wiring arrangement with each socket directly wired to the incoming line pair, and someone in the past has replaced the original master socket with an NTE5, but hasn't reconnected the extension properly.
This isn't necessarily a problem, and there's nothing you can do legally to correct it yourself. But the extension should really be rewired to connect to the NTE5 faceplate. BT will charge you for doing this work, so if you don't have a specific performance problem it's probably best left as it is.
