Here's an image of an ad for one Apparently it's a light-up version of the standard motometer.
Picture off an eBay listing:
If you squint, the small print in the bottom section says "Red Ball Boyce Moto Meter"
I like the other ad text: "...as practical as it is new" is very nice. And of course in the 20s and 30s "radio" was as much of a buzz word as "atomic" became in the 40s and 50s.
I would REALLY love to see one of these in action... A light up motometer? It would go so well with the neon under car lighting that's all the rage with teenagers these days