alternator

My 2008 FZ1 had the rotor magnet failure a few weeks ago and I managed to source a new modified rotor on eBay for 50% list price. I ordered the gasket and O ring for the rotor replacement, but i don't recall seeing an oring on the rotor shaft when I removed it and i can't find one in the garage...

anyone else have issues with the alternator rotor destroying itself? I believe Yamaha have redesigned the rotor but want to charge megabucks for replacement