Hi Chick,
I just looked at the botton of the nut flange that was against the floor and it clearly shows it is dark phosphate. Also, the two 'U' nuts which have their original finish look like dark phosphate. Mine have flange nuts so maybe that was a '67 thing. The threaded shafts are natural. I say phosphate because it is stated here in CRG that black oxide was not the method of choice back then.
Hope this helps,
Mike