Looking at 2nd edition, I would say that is an error code. Page 12 defines the code and does not list 1-IC. Since the list already has one unit of light cavalry listed, it must be something else. Could it be Infantry Cavalry i.e. dragoon?
Or should it read Heavy Cavalry, as heavy is always shown before medium and light?
It doesn’t appear in any other listing. I would be tempted by HC.
EDIT – note the army list for ancients shows 20% Shock and when that happens, the lists always give horse and musket at least one heavy cavalry unit, so that tends to support a view of using HC.