I may be wrong about this but I believe the different tubes were related to engine size.
The 380/420 are small block while the 500/560 are big blocks. The bigger engines need a little more air to idle than the smaller engines.
I know that my 84 500 has the open tube and my spare 90 560 has the open tube. A friend has an 83 380 with a closed tube and another friend has an 89 420 that doesn't have a tube at all, the hoses don't have the holes for the tube. Maybe some one with access can check this out.

__________________
84 500 SEL (307,xxx miles)