I've looked at the black coating on mine. I've wondering if it not a baked enamel finish. If so, it could be the instance where there is no solvent for the finish. Some finishes, once they polymerize, setup and/or are baked, they will not dissolve again, at least not easily. MEK, a shelf brand, is probably one of the strongest thats sold in retail. It's methyl ethyl ketone, an will easily melt plastics, sealers, finishes and a host of other materials. It will even attack many types of epoxy. I will also bet an autobody shop would have a remover for it.
Being powder coating was not around or at least not in wide spread use in the early/mid '50s, for powder coating removal, methylene chloride will typically remove it. But, finding it courtesy of OSHA and other state agency's may be a little difficult.