To add to this, the 'item number' is officially called an element-id and is not only used for a specific part (design-id) + color, but also to different printed parts, multi-color parts etc. etc. In fact, Lego uses the element-id for basicially everything they produce: stickers, instructions, booklets, flyers, promotional booklets, boxes, packaging material. Everything has an element id.
And to Karen: Rebrickable.com has a pretty extensive mapping between Lego part numbers (also the element-ids from instructions!) and LDraw parts
And to Karen: Rebrickable.com has a pretty extensive mapping between Lego part numbers (also the element-ids from instructions!) and LDraw parts