Yes, and my other horns have it too, but just not that long. As I indicated in my OP, the slot in my other horns don't extend past the metal ring/band which holds the neck screw so you can't see them unless you look down into the tube (they're visible from the interior, but not the exterior). This one seems to be several millimeters longer (at least twice as long) and is visible from the exterior, so I wasn't sure if it indicated some kind of damage that caused the slot to crack down the length of the receiver or if this was by design.That's not a crack, that's a slot machined so you can close the clamp on the neck.
You got the answer from turf3. Its not a 'crack'. It is longer than most of them. Possibly a previous owner cut it lower or a shop did for some reason, but most likely it came that way. I would term it a 'manufacturing anomaly'. You could look at it with bright light and magnification to see if it is plated silver on the sides of the slit. That would pretty much prove that it was done before being plated by Selmer. Regarding 'fixing' it, it is not feasible to try it. It would have to be brazed and that would make a heluva mess - really, the receiver would be sweated off the body tube, cleaned up and brazed, than re-plated and soldered back on. Just live with it. If it hasn't caused a crack in the receiver by now its not going to. Also, if you're not having any problems getting a seal on the neck, there is no reason to be concerned about it.That's not a crack, that's a slot machined so you can close the clamp on the neck.