That is a drill bit sharpening gauge, for the common angle at which twist bits are ground. You'll notice that the angled edge (on the left in your photo) has got fine increments marked on it. That will let you confirm that you've ground both flutes evenly. You turn the bit so the bit's cutting edge is against the attachment and note where the "center" of the bit is, then turn it so the other flute is against the attachment and confirm that your center is at the same increment. When the bit's ground at the correct (standard) angle, the leading edge of the flute will touch the attachment over its length, falling away as you turn the bit in the direction it will turn in the drill motor/drill press. Checking the "fall" against the increments on the rule itself will let you confirm that you've ground the two flutes consistently.
There are one-piece gauges like this; these attachments are for machinists who don't grind bits all day long.
If I didn't already own one, you've be seeing a "ping" indicating you've got a private message from me...
