I rate this as the most unfortunate name I've seen in a photo product. Yet it lasted quite some time. Metrawatt of Germany made them (they also made a bunch of other meters, like the Metraphot).

The original Horvex came out just before the war. The Horvex 2 came out around 1955, and the Horvex 3 around 1960. They were also sold in the USA after the war as the Argus L3 line, and I believe they were the basis for the Russian Leningrads as well.

All the main Horvex meters have something in common: a really nifty add-on in the form of an additional sensor. Selenium sells are photovoltaic, which means they generate electricity when light strikes them. The more light that hits them, the more electricity. But also—the larger the sensor, the more electricity, and the more sensitive it is to low light. I believe this is one of the reasons why Westons and Norwoods were so well regarded: they had larger sensors than most.

Anyway, a Horvex's main sensor was a slim rectangle mounted on the front of the meter, like most meters of the time. But you could add the secondary sensor which plugged directly into the circuit and added more juice. The sensor sat perpendicular to the meter and faced in the same direction as the front edge, so both sensors saw the same light. You compensated for it on the calculator dial.

This one has a couple of interesting kinks: first, the sensor is mounted on the back, similar to a Weston. But rather than hold it eye-level, there's a door on a hinge you open and it has a mirror. When the door is open, the mirror is set at 45° and reflects the light up onto the sensor, so you still hold the meter at chest-level. I have no idea what the advantage of this would be, except that the sensor area could be larger than the front edge of the meter. Fine, except that's only a little larger. Not enough to make it worthwhile.

The other unusual feature is that the scale under the meter glass (just above the needle) is connected to the calculator dial, so that when you turn the calculator dial, the inner meter scale rotates with it. So what you do is set the calculator dial to your preferred aperture and direct-read the shutter speed (or frames-per-second) on the meter dial.