I've worked it out myself: On page 3 of the instructions, it says that item_number should be changed to string. I had missed that earlier.
The numbering is to be performed by the Office Connector and the PR Sequence for numbering is added back in the section Create New Document Type on pages 10-11.
Bonus tip for those going through this document: In keeping with good practice, I prepended the new properties for ItemType PR with _ to indicate that they were not system properties (pg 3 of the document). This caused problems later, at least with the external_id property, so I went back and got rid of the underscores.
