The letter designation at the end doesn't matter as long as the primary part # crosses over. Edited!! My apologies ---just saw what you're referring to. The belt you have is for a 20 series symetrical TC. To figure out if its the wrong one, look at your pulleys. if both sides of the pulleys are angled, then it's the correct belt. If the engine side of the pulley is flat and the other side angled, then you need the 591.
I picked up a PIX --e-bay link below ...BTW, since no one posted to your earlier ? about plugs, i'm running a champion N4C in mine.
http://www.ebay.com/itm/320567298053...84.m1439.l2649