Any Dexter can work with any game. It's just a matter of having the proper laserdisc image installed, and setting the proper MODE setting.
Numbers in the top left of the screen, are displayed when the license key gets messed up.
The key is on the USB drive, and is specific to each Dexter.
The most common reason for Dexter loosing its key is if someone has more than one Dexter and they get the USB drives mixed up between them.
Always keep the correct USB thumb drive with its Dexter.
My guess is that Matt will eventually release a final firmware that will not require the key.