The 9155012 Nickel / 9155012B Black is a Two-Module Flush-mount Frame for use with 9155015 flush mount box.
The 9155015 Flush mounted box for walls or plasterboard for 1 module is for use with 9155012 Flush Mount Frame. The box cannot be used for surface installation.
Price quoted below includes Flush Mount Frame + Flush Mounting box.