Slides are permitted because they aren't classified as restricted items. With this particular model of pistol, you're dealing with a lower receiver that houses the hammer and that likely has the serial number on the real steel. You're also dealing with a pistol grip that houses the trigger mechanism and *looks* like an integral gun part.
The fact is that lower receivers are restricted. If you're not sure about whether the part may or may not be a lower receiver, then isn't there a probability that the same thing may occur with the border agent checking your package?