Help asked investigating a feedline coupler

 A feedline coupler, an unusual name for this unusual antenna tuner...

It looks old doesn't it. But what is old? And how old could this tuner be? Let's look at the inside...

This tuner has some wiring and components that are from another century. Does anyone know when?

It is made to tune a open line. The current owner did try it on several bands and it tuned best on 15m. But I guess that's depending on what kind of antenna is used.

There is no diode inside to rectify the current for reading on the meter. In this case a heating element is used. The current owner used this with 100W and saw the meter just going up a little. According to him there should be a lot more input power to have a good reading.

More details:

You can click on the photos to enlarge them.It is a fascinating piece of equipment I think. And...so far we cannot find anything about or like it on the internet.

So I ask readers of this blog to play Sherlock Holmes. I don't know the answers but have questions:

Where was this feedline coupler used?

Is it a commercial build or is it homemade?

Why did they use a heating element?

What year was this made?