There is an Enhancement Request for Department, Class and Location to have Auto-Generated Prefix Number -
Enhancement 146453: Setup > Company > Auto-Generated Numbers > Transactions; Be able to have a check box for Department, Class and Location.
As an alternative solution you may add the Numbers manually before the name.
1.) Navigate to Setup > Company > Departments
2.) Click Edit on the department to be edited
3.) Add the number before the name of the department
4.) Click Save.
This will affect the Transactions created and to be created.