What is the proper procedure (under ASME Y14) that defines how to create a drawing showing modifications to an existing part?

We have cast parts, controlled by a casting drawing. We purchase those castings from a vendor, who makes them to the specifications on our casting drawing. Then we modify those castings in our internal machine shop.

What should this machining drawing look like? Is this an altered item drawing? According to ASME, no, because we are the design authority.

ALTERED ITEM DRAWING (MECHANICAL)
DESCRIPTION & USE: (See FIGURE 4-1)
A drawing that shows the details of an alteration to an existing item under the control of another design activity
or defined by a nationally recognized standard. The drawing type permits the required alteration to be performed
by any competent manufacturer including the original manufacturer, or third party. An Altered Item Drawing shall
not be prepared to modify an existing item that was developed by the design activity.

But somehow I need to make proper reference to the casting drawing. How is that done? This is not an assembly, so it does not get a bill of material.

In this particular instance, there are two castings on the same drawing (multi-detail drawing) as a -1 and a -3. There are three machinings: -1, -3, and -5. So both need to be mult-detail drawings?

If anyone can show me a reference to the appropriate AMSE Y14 standard/paragraph, I would be very grateful. I need to set a company policy and I am currently having a disagreement with a co-worker on the proper procedure. Thus, I am looking for a reference to an authority (ASME Y14).