I've no idea if i'm on the right track, but could it be vacuum formed signage that you are talking about?
That's it! Yeah that sounds exactly like what it would be. But I guess these things are way in the past. A while ago I found a giant cardboard RCA vacuum tube box. These old promotional things are so much fun!
Not quite on topic, but back when I worked for the employer I mentioned above, we sold "Van Ruyten Inverters".
These used an electromechanical "vibrator" to convert DC to ac.
Country people with 32 v DC "lighting plants", wishing to use 250v ac TV sets were a major customer base.
Van Ruyten supplied us with a "counter display unit", which was externally a normal inverter, but unknown to most of us, had no "guts" inside, just some weights.
One day, one of our country customers was "in town", and dropped in to buy an inverter.
The counter guy looked in the store, & we were out of stock, but he remembered the "counter" unit.
"Ahh! said he, I'll just grab that one!"
Off went the customer "happy as a sandboy"!
The Boss was on the phone in his office, & was "sort of" watching these events, but not taking much notice, but his "Spidey sense" started firing when he saw the customer heading off, & an empty space where the counter display unit had been.
Luckily, he knew the customer quite well, & had a good idea where he would go next, so set off to "head him off", leaving the rest of us with the admonition to find a working unit or suffer dire consequences!
Ultimately, all was well, we found an operable unit in a country branch several hundred miles in the opposite direction to the customer's home town, & it was specially delivered to his farm.
The retrieved unit was then clearly marked "dummy"!