Seems to me the black plastic knobs were the end of the run for the solid-tub machines. GM like most US industries in those days was being squeezed to switch to plastics so metals were more available for the Defense Industry. Or, so it was said.
The 1-18's used more plastics, however, for the most part, the knobs returned to metal. After the war ended the metals were more available. The OPEC embargo helped make more metal available as well because Detroit was unable to market the land yachts like they used to.