Advanced Photonix inc. part number PDI-G103 Gallium-Aluminum-Arsenide (GaAlAs) photodiode. It has a spectral response range of 840 to 940nm with a peak response at 880nm. The photodiode can is a TO-46 package. The “emitter tab” is the anode and is commom with the center pin case ground.
The spec sheet is available by clicking HERE