This kind of labeling is widely used as listed below. Is the letter important or trivial? How is it related to the axis, angle, and boundary plane? Thanks.
Σ values denote the ratio called the reciprocal density of coincidence site lattice, which is well defined.
Σ13a, Σ13b, Σ17a, Σ17b, Σ19a, Σ19b, Σ21a, Σ21b, Σ25a, Σ25b, Σ27a, Σ27b, Σ29a, Σ29b, Σ31a, Σ31b, Σ33a, Σ33b, Σ33c, Σ35a, Σ35b
http://pajarito.materials.cmu.edu/lectures/L13-CSL_Theory_GBE-17Mar14.pdf
Σ41a, Σ51a, Σ37a, Σ33c, Σ41c
https://onlinelibrary.wiley.com/doi/full/10.1002/mgea.19
Σ21a, Σ19b, Σ37c
https://pubs.acs.org/doi/10.1021/acsnano.1c06367
https://www.nature.com/articles/s41586-020-2082-6
Here on page 44, you can find a list of many CSLs:
http://pajarito.materials.cmu.edu/lectures/L13-CSL_Theory_GBE-17Mar14.pdf
I guess the letter is to differentiate two different CSLs with the same number of coincident lattice sites.
If it would mean something more, sigma 3,5,7,9,11 would probably also have a letter.
Letter "c" is present only for Sigma 33, and I guess it is there only because there are 33a and 33b.
It is just my guess looking at the table.
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