Singer, rightfully proud of its company history, celebrated its centenary in 1951 by giving all machines commissioned for that year a special commemorative badge. It was the standard Singer emblem of the time but with the inscription "A Century of Sewing Service 1851-1951" around the outside of the oval. These "limited edition" models, commissioned for only one year, are sought after by collectors and will add slightly more value to the machine assuming the machine is in comparable condition to a standard Featherweight.
Most have the 1951 serial numbers but authenticated models with 1948 to 1952 serial numbers have been located, suggesting that the company held vast stocks and simply badged machines just prior to dispatch. The Centennial Featherweights are mechanically identical to the standard 221, so other than the badge there is no other difference.
Centennial Featherweight 221 was $149.50 back in 1951.
Another thing to look for is the inscription - there is another Featherweight model with a blue rim that is very scarce to find. The commemoration is different so you need to know what to look for. Most of the time it is indeed a Centennial Featherweight, but perhaps you will stumble across one of the scarce San Francisco Golden Gate Expo models. Keep your eye out!