This Reddit thread provides a clue that
The firmware devices are listed under firmware, and keyboards for the touch/type cover "firmwares". Their versions can be found by right clicking each respective object and going to the driver tab.
That thread, however, is incorrect for finding Type Cover 2 firmware. The correct device to look for seems to be called Surface Type Cover 2 Firmware Update Device not Surface Touch Cover Filter Device. The full path to that device is as follows:
Device Manager
|
+ Human Interface Devices
|
+ Surface Type Cover 2 Firmware Update Device
Once you find that device, you right-click it to discover the driver version. As best I can tell, the driver version is the firmware version.