To my understanding, the Centennial is really a badge. That is, not a distinct model, but 221s that were the same as the others, but badged with the Centennial badge. The serial numbers on Centennials can be pre-1951. There are also other special event badges, like for state fairs, etc. I have a Centennial, and it appears to me to be identical to my 1952 & 1953 models, except the faceplate, scroll pre-1951, mostly striated after, though there are legitimate reasons why a machine might have the "wrong" faceplate for it's age.