The 1851-1951 badge for the Singer 221 is usually NOT available because it would have to come off from a machine that had one. Sometimes there are machines that have other issues that can not be saved and so motor, etc are removed and sold separately.
I have NEVER seen the metal badges for sale anywhere. They are riveted on and it would be a big problem getting one off still intact and putting it back on another machine still without bends or crinkles and using rivets would be a big issue.
Not all machine made in 1951 had the 1851-1951 badge. It is generally accepted that the gold badge with the blue oval became available in around 1950. So, you will see some 50's, some 51's, some 52's with it.
But, I have a single owner, bought new that was made in 1948 that has the 1851-1951 badge. They refilled the parts bind as needed and just reached in and got what they needed so, you will see 1950, even a 1952 with the 1851-1951 badge.
An earlier reply was correct that the badges that are available for sale are the ones that were a decal with a big 'S' that came on the white (pale turquoise) models from the 1960's because they were not as sturdy and did get damaged and are often missing. As far as resale, you have the group that wants the machine to sew and the group that wants the machine for display. Some of us love them so much and we want our cake (machine that looks good) and eat it too (machine that looks good to sew on).