While working at an English University 30 years ago, I had it drummed into me that "c." at the end of a letter stands for "copy to" and cc. is the plural when a letter is sent to more than one additional recipient. This was compared to f. "and the following one" and ff. "and the following ones". Does anyone else still write c. for "copy to" and cc. only for "copies to"? - or does it count as wrong these days?