[DB Design] Sometimes-Tracked Stock

Hi, my question is about database design. I'm trying to design a database for a stock control program as part of a larger system. The problem I am having is that some items in the database have individual serial codes, and others don't (all items have a barcode to indicate which item is it).

I've simplified the details here, but here's what the item table might look like if no items had individual serial numbers:

And here's what it might look like if they all had serial numbers:

Notice absense of CurrentStock in Items, as this could be computed, also, quantity of a particular item assigned to a particular job could also be computed

My problem is that while some items are individually tracked with serial numbers others are not, just a note of quanities is used. I can't figure out a comprimise between the designs which allows for this.

Here is a design I'm thinking of:

Unfortunatly this has several dangers to data integrity and consistancy. Can someone please point me in the right direction with this design? Thanks.