It's widely used across many builder trades.... carpentry, tiling, plastering, boilermakers..... it's also a very "old" tool in origin!

Sounds like you know what it is, Janx. Why don't you go ahead and guess?

ohhh, well, then it goes to Elmojo as he had it first, it is a square after all

I use it for woodworking.

Please post a pic from a different angle; sounds interesting.

Thing don't look square to me 

It's folded.

http://i.imgur.com/fdDq2HK.jpg

Ok, back to me then?
Give this a gander....
right here

How big is it?

It's just an adjustable carpenter's square. You can set it to any desired angle to mark a piece of timber for an angled sawcut.
When I was a kid there would be one of these in every man's shed! (Jeez I'm not THAT ancient are I?)
Bluddy'ell if you think this is a mystery I've got a few that'll strain a brain!

So it's the same as the last thing i posted ? ))))))

Wait, I'm confused...
Axe, are you saying that the link I posted looks like your square?
My new mystery item should be shown in that dropbox link. Let me know if it does show right.

No, Joolstacho said that your item is an adjustable carpenter's square, just like mine. Hmm, now that I think about it, maybe he was referring to my item )) Sorry for the confusion ))))

LOL right!
Yeah, mine is definitely not a square. In fact, it's not really a tool at all.

Quote from: Joolstacho on September 04, 2015, 12:30:44 AM
It's just an adjustable carpenter's square. You can set it to any desired angle to mark a piece of timber for an angled sawcut.
When I was a kid there would be one of these in every man's shed! (Jeez I'm not THAT ancient are I?)
Bluddy'ell if you think this is a mystery I've got a few that'll strain a brain!

I've got one, just not that exact design.
I also wouldn't call it a square. It's not a square. Squares make 90* angles, some times 45* angles.
That is a bevel gauge, also called a T-bevel, a sliding bevel gauge to be specific. But not a square.

T-Bevel! That's the word I couldn't think of earlier. 

Anyway, any guesses on the new item?

a flip up sight?

Sorta, but probably not what you're thinking...

You folks give up?
Do I need to give a hint?

hints are helpful

Slack was on the right track with the sight guess, although it has nothing to do with guns or firearms of any sort.
What else needs to be "aimed" to function correctly?
Another hint: It's very old. Not 1800s old, but 50s-60s I'd say.