Very true, these are the original belts on my 61" flip deck. I am very lucky to have them last this long, but that one area is something I've never seen before. I cant think of anything that would cause wear like that, so my only thought is it was a mouse.
18 years on a V-belt, pretty good wear I think. See some cracks in the belt and probably separated, got opened up on a reverse side idler, caught something and tore out the chunk. Never seen that before though. Might run another season or two even so.