This is a miniature trackball with select. It's essentially a series of normally open switches that close based on which way you spin the ball.  Spin the ball in the +X direction and a switch periodically closes between Xscan and XO, spin it in the -X direction and the switch closes between Xscan and XO', with the same sort of behavior on the Y axis.  Push the ball down for a closed switch between the select lines.  Very handy interface to all sorts of miniature electronics.

Part # : TBWB2A00

Breakout board available below.