Does anyone know what this old tool is and what it is used for

[cliffkenn-3]

This tool has an arm which can be turned from one side to the other by rotating it into the handle.  Ends are as pictured.  Handle is wood and arm is metal.  There is a mmagnet with is attached but not sure it belongs to this item.

[safetyfirst5-91]

Vintage Sliding Bevel Hand Tool

https://www.ebay.com.au/itm/165644814669?itmmeta=01JB4V2WX049CEM73KVK43W9AK&hash=item269133494d:g:qT...

[eburtonlab]

If it is not obvious from the previous response, this tool is used by carpenters to copy and reproduce the angle of an existing cut.

https://www.youtube.com/watch?v=nzQYd8wxCQE

[redmodelt]

Could be used to mark something at any angle needed. If I needed to make a 33-1/3rd degree cut, I could set the angle on the tool and make repeated marks for cutting. 

[stephenmorgan]

A modern digital version 

Digital Sliding T-Bevel/Angle Finder