I thought there was some kind of photocell there! I learned that from when I first tested 1 of mine and the LED didn’t flash. Turns out it wasn’t an LED after all! And yes the Light Test units came out around 1993 or 1992. They use the 2nd generation SA67D cover and base, and have the newer sensor and piezo horn with only one hole. They do the 4/6 alarm pattern.
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