[lame question] Something on the back of the head - what is it?

[kavoo]

I've seen it several times on photos and always wondering what the heck is this steel 'thing' attached on the back of the head (circled on the photo below)? I've tried to find something on Google, but - since I have absolutely no clue what is it and what it does - not surprisilgly haven't found anything...

Could any1 enlight me?

 

[guitarmanUK]

I think it is normally to hold the Allen keys used for the lock nut

 

[Hubertus]

It's an Allen key holder

 

[kavoo]

So simple... and I was already starting to think it has something to do with sustain improvement like Fender Fatfinger

Many thanks mates