That's a Nato type socket (female) and powers up anything that you have suitably wired with a male Nato connection. A little like the tow ball electric socket. Same but different. It's a universal type socket used worldwide, more popular in some places than others. (armed forces etc).
will give it a go laterNow - hopefully you can open it. Some owners have had problems with that, even using channel locks - so good to put in a little anti-seize compound