Ball Bank Indicator, Model: QM III
This version is mounted in a case that fits into a standard 57 mm (2 1/4") instrument hole.  See drawing at right for dimensions. 
Whenever there is no turn and bank indicator, it is advisable to install a bank indicator.  This simple instrument is particularly useful in training aircraft.

Note: There is a small bubble inside the glass tube.  There is a small space for the bubble to hide on the right end of the tube.  If you see the bubble then simply tilt the unit to the left until the bubble goes "home".
Dimensions