My guess is that it was replaced at some point. Back in the 80's I went to the local auto supply to buy a replacement alternator for a daily driver 65 Mustang with a 289. The original alternator had a single grove pulley. The guy plunked down a rebuilt alternator on the counter with a pulley that had 2 grooves. I said what gives? The original had a single groove pulley? He relied, "don't worry, it'll fit". I went home, installed it and it worked fine. I just replaced that alternator with the correct one about a year ago.