The Tyco MR901M (516.057.041) is an Optical Smoke Detector, and is part of the M900 Series of Analogue Addressable Fire Detectors.
The Tyco MR901M can be plugged into an M600/M900 Universal Base to form an analogue addressable Detector. The MR901M is designed to transmit an analogue signal, which represents the state of the sensing chamber.