Serial Number items are maintained in a separate file.  A permanent record is created for each Stockitem/serialno/loccode when the item is issued and made part of a higher lever assembly or otherwise moved, the record is never deleted, the quantity is just changed to 0.

When attempting to issue serial number items to a WO the whole history of serial numbers at the location is displayed in the selection box.  A check in the SQL for a quantity > 0 is needed.