The easiest way is to have a couple of helper columns as rightmost ones in your table (I see you have defined your table as Table, so formulas will expand automatically whenever you add new entries). Later it will be best you hide those helper columns. And in case Table formulas are used, this will be easier when you don't use spaces in column names.

To continue from here, some additional info is needed:

1. What do you mean with 'last S' ? Is it in your example [No.] = S1459, or [No.] = SC1532? I.e. are you grouping data by 1st character, or by string part of [No.];
2. With 'last' you mean lowest position for given group, or biggest numeric part after string part (when later, then you get same result with any sorting order in applied to your Table);
3. How many characters can string part of [No.] maximally have.