It had occurred to me that it might be as simple as that, that one of these in your box meant you always had a level, and what is the most likely strait edge a carpenter might have to clamp it to, his square.

I just have the one. It’s un-winding I’m talking here. The one gets it done.

They are accurate despite the seemingly short registration. No adjustability – The screw heads you see in some of the decorated brass tops are not real but just part of the design.