This web site is an interesting read and could be helpful.
http://singer301.com/limited/default.html
In 1951 Singer produced two special classifications for the 301: Centennial Limited Edition and Anniversary Limited Edition. Centennial are very rare, only 1,000 produced and they have the centennial badge. Anniversary not so rare, 9,000 produced (4,500 black) in July 1951, but only a few hundred found. There is a raised ridge on the body around the new black border badge, and it has the "paperclip" decals.
If you are a purist; is it a centennial machine because it has a blue centennial badge, or because it was actually produced in 1951? Singer did not start marketing the 301 until October of 1952. When you say there is nothing special about machines built in 1951 I have to take exception. The awesome 301 machine marked a significant shift for Singer for the second century of production and owning one of the first ones produced seems special to me. Also I was born in April of 1951 so it makes it extra special.