This I a flat backplate meant to replace the back plate with the mount on an Accusize Dial Indicator P900-S102, and P900-S001 (thanks @JK47TheWeapon.) 

This is meant to replace the back mount and allow the dial indicator to fit in places where the right angle mount would prevent it from seating properly. Instead the dial indicator needs to mount via the collar/needle sleeve. This could also be used as a base to design a custom mounting option.