[Arran]

I think that the bean counters at Philco probably wanted to build the 66MB cheaper then the 60MB cabinet of the previous year, so cheap plywood and dark tone spray took the place of ribbon mahogany veneer on the top, and sides, all in all probably saving them $.05 per cabinet. I don't know whether it is fir plywood or not, it could be poplar, or some sort of pine, the only way to tell would be to strip off what's left of the finish and sand it, Doug Fir looks sort of pink when fresh grain is exposed. Ironically if it was fir ply, it would have turned to an almost burgundy colour by now if they had only clear lacquered it, which would have made a nice match for the mahogany on the front. If it were fir then a baking soda solution will darken it to that burgundy colour. 
Regards
Arran