You cannot have the policy serial no under the policy tab.. its just a template
Right click on any client click on properties under client tab you wil fnd the policy serial number.
to know what policy serial number your cleints are using, you can check any of these.
locate any group ,say default
click on deatails locate the policy serial number.
now go to
F:\Program Files\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent\60DF7F180AB08B310005354AA7DC62E8
you will find the folder with policy serial number what you noted above
in this folder open the profile.xml
you will find a diff serial number, if your client has changed location..