Odd size to be sure, I was initially thinking C-body wagon or maybe Imperial. I guess it could be a drag wheel, though I seem to remember even way back seeing mostly 15" on the serious doorslammer drag cars. By "serious" I'm thinking of the faster cars that'd run SS, MP, etc. But by then in SoCal those guys tended to run aftermarket wheels, not steelies.
My other thought is whether it was something Scott Harvey pushed through for the rally program? That would require a 4.5" conversion on the A-bodies of the time, of course.