If your OS X Server has died and you need to rebuild it, you may find that you haven’t kept good track of where your serial number is.
Also nicve to have when you need to make sure you are not using the same serial twice if you have more than 1 server.
Nathan Toups posted a solution for that in his Twitter feed.
To do this on a running system:
1. Log in with an admin account.
2. Open Terminal.
3. Type sudo cat /etc/systemserialnumbers/xsvr
Your OS X Server’s serial number and the registered name and organization should be displayed.
If you’re retrieving from backups, get the xsvr file from /etc/systemserialnumbers to retrieve the serial number.