Oh okay and I was wrong about never hearing piezos when I was really young but it was because I wasn’t sure if it was a piezo or not because it didn’t pulse. Again we are here to learn. My parents had a square shaped Sunbeam unit in the crawl space of their old house. It was not a Sunbeam centurion, it was actually called a Sunbeam smoke sensor and the alarm was from about 1982-83. The name “Sunbeam” appeared in raised lettering on the cover of the unit and the test button was clear and also flashed every thirty seconds when the unit had a properly working battery in it. This alarm also was smaller than many units of that time period. You may know the alarm Im talking about. Im pretty sure Fire Alarm Fan would know what it looks like. There was a round Dicon unit around the same time of this one and I believe it did a continuous piezo as well. The Dicon resembled the unit and had the same kind of test button.