In a previous question here What is the proper way to write the plural of a single letter? (another apostrophe question) someone asked what the plural of a letter is. The answer given was for uppercase letters use a lower case S with no apostrophe, but for lower case letters use an apostrophe for clarification.
What about when using the letter S? In either case, having Ss or s's looks odd. Would the plural be Ses or S's?
And when using uppercase letters that can become a word, such as Is and As, is no apostrophe okay or should an apostrophe be added for clarification?
This may be marked as duplicate for Plural of an initialism that ends with the letter S or similar questions concerning acronyms ending in S, but I believe this is different because I am asking about the letter itself, not an acronym.