If you need to get the serial number of an HP server, and the HP Tools are got showing it, or you want to script it, you can use the WMI command line.
Launch Powershell as administrator, then type in the following:
wmic bios get serialnumber
This should display the serial number of the server.
If you would like to get the serial number and model, launch a command prompt as administrator, then type in the following:
wmic csproduct get name, identifyingnumber
This should display the serial number and model name.