I believe so. Fender changed their plating a couple of times during the early amp days. Some early tweed and early 60's "front mounted knob" amps appear to be nickel plated, the later ones seemed to be zinc plated. They are the ones which get a film of white powder on them, when the zinc leeches out of the metal. Its a mild form of corrosion.
They clean up nice with 000 steel wool and a little WD-40, which leaves a protective film behind. If you have them completely stripped of all parts, they can polish up well with car chrome cleaner too.
Thanks Andy...totally the info I was looking for. I have a 66 Pro Reverb with a white-ish film all over the outside of the chassis. I suspected it was oxidation by way of the zinc plating. I have a '65 Princeton Reverb that is still very shiny which I beleive is Ni plated.