One setting on the voltage range is 1MV. What is that thing?
In the olden days some US vendors did not fully appreciate that SI units are case significant. milli and Mega must of course be discerned by case. hp was also responsible for such misses on some devices.
As to what it is, it is a valve voltmeter or "vacuum tube volt meter", VTVM for short. The classic model everyone else tried to copy is the hp 410b which is much more competent than this Eico. Not that the Eico is bad; the 410b is simply exceptional, with AC measurement up to 700MHz, with DC and resistance ranges to boot.
Why was there such a focus on them being valve voltmeters? Well, for starters, the only electronic amplifier available was the valve. Very early VTVMs started appearing in the 30s, and were perfected in the early 50s. And, with the alternative being electromechanical meters like the Avometer or the Simpson, where, even in those quality instruments, there is a low resistance in the meter (affecting the DUT and the reading) and bad AC frequency response (most likely not above 20KHz, and even that was a stretch) it obviously was important to point out things like a DCV resistance in the range of 10MΩ and AC response up into UHF.
I've wanted one since I read the service instructions for 60s HiFi gear stating "you need a VTVM to get this right, don't try it with a purely mechanical instrument". As a consequence, a lot of gear was shipped with service manuals that said "if you're using an Avometer 8, this is the value you should be seeing at test point 46" e
You can get a cheap one like the Heathkit VTVM for perhaps $50, with lots of variation in price. I've got an ex MoD Marconi. Temperamental, but works.
HP410C is my favourite!
We used them, back in the day, along with the hp coaxial sampler to measure power on lower power TV transmitters, using V^2/R.
The good thing was that the probe was a "sample & hold" type, so we could read the RMS value of the "sync tip" voltage, & directly calculate "sync tip power", as that was what the specification for vision/aural power was specified w.r. t.
I picked one up a couple of years ago at a Hamfest--- the "ac zero" pot was jammed, but a bit of "working " of it back & forth cleared the problem.
As is common with old ones like this, the centre spike from the DC probe is missing, but everything else works well, now.
As to using an AVO 8, both Tektronix & Marconi often built in voltage dividers to a test point, so that critical voltages could be checked with an AVO, Simpson, or similar VOMs.
A.T.E. tube type VHF point to point Radio links provided special test points so that grid & anode currents could be read with an AVO 8.
All good, except if you are trying to "peak the grid & dip the plate" at a minesite, where the primary function of the power station was to power an enormous machine which, every time it dug into the ore face caused the Mains to dip alarmingly.
You would be just approaching an anode dip, when the lights would suddenly dip, & the AVO would go crazy!
We had Stabilacs, but the voltage drop was too severe & too fast for them to handle.