This part is a kind of semiconductor called 3DU5C.

Function of this product has Metal Encapsulated Silicon Phototransistor.

Manufacturers : ETC

Image and pinout :

Some of the text within the PDF file :

3DU5C Metal Encapsulated Silicon Phototransistor Dimension Specification  NPN Silicon Phototransistor;Model : 3DU5C  Working Voltage(Max.) : 10V;  Reverse Breakdown Voltage : 15V;  Dark Current : 0.3uA  Photocurrent : 0.5-1mA;  Power Consumption : 30mW;  Peak Wavelength : 880nM  Body Size : 7 x 5mm/ 0.28" x 0.2"(L*D);  Total Length : 28mm/ 1.1";  External Material : Metal  Weight : 3g; [ ... ]

3DU5C PDF Datasheet Download