I suspect this is some kind of phototransistor, except it has three leads instead of two. It has no part number anywhere on it. Is anyone familiar with it?
Photo transistors were often regular transistors exposed to light so were fabricated with normal transistor die geometry. For many applications the base connection was not used so was not brought out of the case. In some cases base bias or switching was used to adjust speed or sensitivity or provide external inhibit, test or override and was made available.
Some opto-couplers still have the base connection available though it is seldom used in digital applications.
Check out Figure 26 in the Photosensitive Devices showing the 3 connections used.
The package was a TO-106 and was more popular once upon a time.
You can check out this Google picture search for a number of examples and follow the links for some contemporary and vintage part numbers.