Here are some thoughts i had regarding the materials of the keys:
Middle key is silver, Bottom key could be gold or bronze, top key seems to be iron. Initially i thought top key would be gold but the enigma himself hinted at the rust on the first key.
I looked a bit into the greek "ages of men", since they seem to be derived from a poem called "Works and days" from Hedios, where we have a golden, silver, bronze and iron age (as well as an age of heroes. However i haven't exactly figured out whether this leads to anything and i might be totally off anyhow.