If you line up the Scott 220, 220a and 220c (or illustrations of same) and then study the Arabic numeral 2 you will notice that they are not consistent.
On the 220a the left number has a "cap" which is a small flat blob on top of the uppermost curve of the 2. The right 2 has no such protuberance.
On the 220c, both the left and right 2s have this "cap."
On the 220, neither 2 has any top protuberance.
Here is a 220a (not mine) now listed on Hipstamp which shows the left cap clearly. It is kind of a flat top sticking up from the curving arch of the 2. Contrast that with the appearance of the right hand 2. If this were a 220c, then both the left and right 2 would look like the left.
Hope this clarifies.