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 Centennial badged machines 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. Thus, the date on the dating charts may be different from when it actually rolled off the assembly line. For example, in January 1951, at the Elizabethtown, New Jersey factory, there were 50,000 machines assigned a block of numbers. The factory most likely was already planning their next batch based on demand and production rates. We do not quite know the lead times between block assignment, actual production, and the next block assignment, but it is probably safe to assume that your machine was produced sometime between the block assignment, and a relatively short time before or after the next block was commissioned (because lead times are unknown). The Centennial badged Singer Featherweight commemorating the 100-year centenary (1851-1951), Singer was probably eager and assigning the special badges to machines they thought would be sold that year. This is why some older machine has a 1951 badge. However, as the centennial celebratory year was coming to a close, the badges would have gradually been phased out. Standard parts were put into bins and would have been filled regularly along the assembly line, so depending on the stock levels (and how fast the factory worker was), there could have been quite the mix of old and new parts as the changeovers were taking place.
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!