I can't help you, but you might see if you can find it listed somewhere. If so, there might be a different part number that superseded it, or that it replaced. Then by looking for that part number, you might find it.
If you can get a parts diagram, you might look for related parts, and check e-bay. If they have part X, used, they might also have the part you need.
Another thing is seeing what you can get, and find out if it will work. Sometimes the difference between an existing part, and an NLA part is color, length of knob shaft, or connector size/location.
You may have to dig of 1978, but with patience you could come up with something.