I have seen them and I have installed them They are "Rate Of Rise" heat detectors. They could also be "Fixed Temp" heat detectors. You would use a fixed temp in the attic, because a Rate of rise, might go off on a hot summer day. They replaced another type of fire detectors. ( I wish I had pictures of them.) The older types of fire alarms relied on copper piping filled with air. The "Heat Sensor" was a bigger copper tube connected to the smaller piping. This whole piping system was pressurized. If there was a fire, the pressure would build up in the pipes and set off the fire alarm system. They were very difficult to trouble shoot if you got a leak. You would have to walk around the whole building with soapy bubble water. You would spray the bubble water onto the copper pipes and look for leaks.
