Nice machine! I'm still looking for my 301. I had a chance to buy one, at a reasonable price, but flubbed it.
As I understand it, the centennial machines were not necessarily made in 1n 1951. Some were made earlier. Not every machine made in 1951 (the year the serial numbers were allocated) is considered a Centennial machine. They have to have the Centennial badge. It was a marketing campaign for Singer in 1951. They did a similarly badged machine for the Texas centennial in 1936 (red badge). I've heard that they sold the machines at the state fair.
I've seen photos of 15-90s, 15-91s and FWs with the badge. I just did a search and found a photo of a 301 Centennial badge. It's shown below. It looks like the other ones I've seen.
bkay