This voltmeter covers up to 20 volts. Once zeroed, I found that it tracked very precisely with an applied voltage as measured on a DVM. What is unusual about the meter is the pointer which manifests as a floating triangle of light projected onto the vertical scale from behind.
The light source is a 6 volt incandescent bulb. I power it with a 6.3 VAC transformer rated for a couple of amps. I didn't measure the actual current but small power transformer gets quite warm! The bulb is very bright, making the meter difficult to photograph. In normal operation the meter would be in an enclosure and the light would be hidden except for the arrow image projected through the translucent linear scale.
This is what I saw after removing the numerous small screws from the side panel.
The light bulb is positioned above the image and passes through a crisp metal mask that forms the pointer arrow shape. The beam reflects from a fixed mirror on the opposite side of the case. The reflected light then passes through a focusing lens and onto a mirror that is attached to a typical galvanic meter movement. This tiny mirror rotates with the shaft axis of the meter movement. The change in angle dictates on what portion of the linear voltage scale the pointer image falls.
To me this is a beautiful device. I must find a use for it. Unfortunately, it wastes a lot of power in that bright bulb. Either I must find a very occasional use for the meter or find a way to reduce how much power it uses. The scale would be perfect for measuring voltage from a PV system, but wouldn't it be ironic if the meter consumed all the power from the solar cells.