Firstly, doing any sort of mask attack on 24 characters is really not possible. Even on the fastest hash, that would take trillions of years. On passwords of that size, you need some really good wordlists and rulesets.
Second, you seem to misunderstand the mask concept. ?a means all lowercase letters a-z, all uppercase letters A-Z, numbers 0-9 and the subset of special chars in ?s is tried on that given position. So ?aC would try all of the above followed by an uppercase C. I'm not sure where you get that C from in your question.... Maybe you are confusing masks with rules? You can't use a mask attack with rules. Unless you pipe hashcat into itself. But you should really familiarize yourself with masks and rules....
Second, you seem to misunderstand the mask concept. ?a means all lowercase letters a-z, all uppercase letters A-Z, numbers 0-9 and the subset of special chars in ?s is tried on that given position. So ?aC would try all of the above followed by an uppercase C. I'm not sure where you get that C from in your question.... Maybe you are confusing masks with rules? You can't use a mask attack with rules. Unless you pipe hashcat into itself. But you should really familiarize yourself with masks and rules....