One one Uno:
USB:
5096
5096
5120
5120
5096
5096
5096
5096
9V in:
5073
5073
5073
5073
5073
5073
5073
5073
On another one:
USB:
5028
5028
5028
5028
5028
5028
5028
5028
9V in:
5006
4984
4984
5006
4984
4984
4984
4984
4984
I don't think you will be able to get the "magic number" using this method. Not for a series of boards.