I have a carb spacer on the box it says d3tz 9a589 J, on the part it says D7PE-9A588-CA. So what is this thing really for? I am aware that Ford puts engineering numbers on the part but why the 4 year gap?

 

Numberdummy will be in here shortly, with your answer.

 

The part number is on the box, the numbers on the actual item are engineering or casting numbers. Since you new part has a 1979 casting number, chances are it had been upgraded over the years. Most likely due to the fact that the original spacer was aluminum, and often had corrosion issues, many later spacers were made from cast iron.
Another good example if the FE cylinder head, casting number C8AE-H. These casting number is used on a variety of configurations, (8 & 14 bolt exhaust bolt patterns, with & without Thermactor smog pump provisions) all these versions would have different part numbers on the box, but still carry the same casting number. Also the part number would be different for a bare head vs. a complete head assembly. Even something simple like a spacer either coming with or without a gasket or small hardware piece would require a different part number.