Century was a trade name for cast iron cookware made by the Birmingham Stove, or Birmingham Stove and Range Company of either Martin, Alabama or Atlanta, Georgia respectively. Birmingham took over Atlanta, formed Birmingham Stove and Range then went belly up because they stupidly misread the market and tried to make it on "back to nature" wood burning stoves. It seems the hippies were too unreliable and the surivalists too suspicous. Too bad, as they made some good barbecue grills.
Your kettle is probably mid to late XXth C.
Rust is like cancer. You need to take care of it ASAP.
If you want more, you'll have to do your own research. Restoration and care info is easy to find. There are cast-iron collecting societies, and undoubtedly scores of thousands of people who can tell you everything you want to know about the maker and the particulars of your kettle, but you may actually have to do some snail mail and/or faxing.
BDL