Is it Cad or Zinc? It seems that some were Cad Plated and some were Zinc. It's been argued about for years. I can only tell you the nuts,bolts,washers and other small parts I took to our Materials Lab were Cad Plated. {when I say "our", I mean Honeywell Space Systems}. They have several ways of identifying the specific compounds of any materials, and thus specify the proper handling and disposal procedures for recycling. I'll take their word for it. Other people have had parts tested in one way or another - and were told it was Zinc. Cadmium was commonly used in the US and around the world in the post WW-II period for corrosion resistance, up until it was classified as a Hazardous Material by the EPA { a heavy metal}. Which in turn subjected the Cadmium and associated plating solutions to expensive control, containment and disposal fees, as well as to EPA harassment. Today other than a few Aerospace and Defense contractors or their suppliers - not many Plating companies will use Cadmium for anything. The Plating Shops that can use Cadmium, will charge about three times as much as Zinc plating, because of the additional costs of regulation. {which is not a problem for DOD or NASA}. Shops that can't or don't offer Cadmium plating - will tell you it is the same as Zinc. Indeed the finished product can look almost identical. Also over the years the technology of electroplating has advanced and different compounds with Zinc can be almost as good as Cadmium in terms of corrosion resistance, but it takes very sophisticated plating control processes - and again it is more expensive. Cadmium is about three times more durable in corrosion resistance than Zinc plating of the same thickness. Parts plated with either Cadmium or Zinc come out a dull sliver in color. The Bright Yellow finish with blue/red highlights is the result of a Yellow Chromate dip. That is another sacrifice coating applied over the top of either Cadmium or Zinc. {kind of a surface sealer if you will} Given the appearance of nuts, bolts, washers found on now 40+ year old Z's - I'd have to believe that the guys in the Materials Lab were correct, and that most of the parts were originally Cad Plated. Zinc plated parts simply do not last that long. FWIW, Carl B.