The "odometer" is a separate, small, self-contained circuit board. It includes the display, the memory of your current miles, and the authentication required by the computer to allow your car to start.
When swapping instrument clusters, you should keep your original "odometer" circuit board, by swapping it from your old cluster to your new cluster.
BTW, what year Cobra? It might not be really compatible. The tachs are different due between the 2v and the 4v engines. The newest ones have a boast gauge. And over the years, the idiot lights along the bottom have changed. Also, even though they look quite similar, it could be a mechanical speedometer or a electronic speedometer, too.