Its 1923 and the phonograph has been around for 45 years, and local radio has been around for 3 in some areas, so I think they would know what an entertainment device is. A tracking device, like a radio that tells people where you are without your input. These idea would not go over the heads of any citygoer, though you would struggle with any back country folks.

I agree I should have phrased that better. Something like a "secret agent following you wherever you go" maybe?

Maybe a communication device? That's how it started at least.