A record list based on which table?
If you base the layout on your line items table and specify either a list or table view of the layout, you'll see all the records in your found set.
If you perform a find or Use Go To Related Records, you can list all such records for a given Purchase Order.
This is what View Purchase Order is intended to do. However, I've just spotted a bug in that starter solution. If you change the layout's view to List instead of Form, you'll see all the purchase order items in a list. (It previews and prints correctly with either view option, this only affects browse mode.)