I recently acquired a box full of miscellaneous electronics components (thanks, Steve!) and am in the process of identifying everything. I'm done with most of the semiconductors but there are a couple of items that I'm having problems with. The picture below shows one of them--it looks like a transistor, but it's not. I figured it's probably a voltage regulator, but I can't find a number that matches up with it. Anyone here have an idea? The lettering reads ZEN 315.