What is the name of this sliding collapsing square tool?

Question

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)

Answer 1

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.

Answer 2

That looks like a pantograph. Used to enlarge or reduce drawings by tracing the original.

https://en.wikipedia.org/wiki/Pantograph

Answer 3

I think you might be looking for some variant of a double try square.

A try square is:

a woodworking or a metal working tool used for marking and measuring a piece of wood. The square refers to the tool's primary use of measuring the accuracy of a right angle (90 degrees); to try a surface is to check its straightness or correspondence to an adjoining surface.

[...]

A traditional try square has a broad blade made of steel that is riveted to a wooden handle or "stock". The inside of the wooden stock usually has a brass strip fixed to it to reduce wear. Some blades also have graduations for measurement. Modern try squares may be all-metal, with stocks that are either die-cast or extruded.

I haven't been able to find a picture of a double try square, with two L-shapes linked together, as you're looking for, but here are various images of single try squares taken from here:

The second of these is called a framing square and the third a bevel square.

A machinist square is the all-metal equivalent of a try square. From Wikipedia:

A machinist square or engineer's square is the metalworkers' equivalent of a try square. It consists of a steel blade inserted and either welded or pinned into a heavier body at an angle of 90°.

More pictures, from various sources:

Hopefully at least some of these look similar to what you're hunting for, or at worst will give you some helpful search terms!