Hack Kit Pro mystery part

I was finally getting around to tinkering after the holidays with my Kickstarter hack kit pro kit. When I took some of the bits and pieces out of the case to look at them I was able to figure out what most of the items were, or they were large enough to google.

Except this little guy in the red circle. Through deduction on the BOM it might be a ‘photodiode’ but I am unable to find schematics or a how to.I found something close but it was attached to a breakout. I don’t want to blow it up. It has a small E12 stamped on the back.

Any help figuring out how to use it or what the heck is is?