What is the name of this sliding collapsing square tool? I am trying to find a tool I had a long time ago. Uses probably range from locating to measuring. Could be used for anything really, drawing to wood carving.

The device is expandable. In its initial position it would look almost like a + ; when fully expanded it would look like a square. Basically it is two pieces of L-shaped metal that slide, but are joined with two sliding rivets:



(red represents the sliding rivets and the left image represents fully closed, while the right represents an expanded state)
I think what you have is an adjustable viewfinder, a tool used by artists to help frame a scene.

By sliding the arms along the guides you are able adjust the view of the scene you are trying to capture.

Helps the artist to learn how to control the content, design and layout of an image.

Here's an example from the Derwent company. 

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Here's an example of someone using their fingers.