When you talk about levels of flatness, you're really talking about the levels of gloss or reflectivity, which can range from a mirror to a cardboard box. If you had a reflectometer in your toolbox, you could measure your paint's reflectivity and rate it on a scale from zero to 100. Paint industry guys love doing this and have specific terms for the various levels:
Less than 10 Flat
75-90 Reduced Sheen
More than 90 Full Gloss
Maybe the paint is the same as the firewall blackout, but the firewall was painted in the blackout booth and thestripes, rocker blackout and tail panel blackout was done in the inline repair booth, requireing both gloss black and whatever this tail panel blackout is to be running in the paint shops manifold system. Just not something that was necessary from a styling point of view and not something you nwould expect to see in a manufacturing process. Its not like the flat black that was used on the tops of hoods of the early 70 mopars like the T/A challengers and AAR cudas which had a purpose, A to keep the reflections down on the hood, B the hoods were fiberglass so it cut down on paint prep time, and C there was a specific style they were looking for.