What do you call a non-digital billboard that constantly morphs between two different ads? I am not even sure if the way I phrased my question is correct, but what do you call these dashboards? I know the ones we use nowadays are simply called digital dashboards, but I don't know how these old dashboards are called.
This is the kind of thing that regular people don't often refer to and so there isn't a common expression. I suppose "rotating billboard" (or "rotational billboard"?) would be understood in context.
There is (according to Wikipedia and a manufacturer a technical word used in the advertisement industry: "Trivision". Individual companies that make these signs have their own brand names, such as Prismvision or Tri-face.
There's an uncommon, technical word for a changeable display made out of rotating triangular prisms and it's
These are ancient Greek mechanisms and are the inspiration behind Trivision (as noted in James's answer).
I couldn't say whether Trivision or periaktos is a better choice in terms of clear communication, but it's nice to have options :)