With just a cursory look, I'd say it was spray. If it was black nickel plated the interior of the shell would also have some color to it. When plated the shell is dipped in a bath and the interior gets the same treatment (but without the polish/finishing). If it's sprayed, then the interior of the shell will look like the natural material as there is no reason to spray it (and it would likely turn out uneven due to the complex curves of the bead and flange areas). Here's what looks like a more modern BB interior for reference.

I'm going with black lacquer also. A common finish at the time.