Create a text file named invoice-numbertxt in the folder. Make any necessary changes to the numbered field for example adjust to the left bold etc. In Word 2003 choose Insert from the Table menu and then.
Then remove the number list you have inserted in the step 2.
Insert an SEQ field. In this video I demonstrate how to generate a new number in sequence starting from whatever number you want. In the Insert Sequence Number dialog box click the New button. Type SEQ and a space.