I do remember reading about it a triumphrat.net prior to purchasing my Rocket. I believe Triumph changed out parts mid-model for a beefier gear. Unfortunately when it goes it has potential to do some significant collateral damage. Though I think it was probably corrected early on so it isn't a "common" problem.
Bike Bandit lists two numbers for that part (which confirms a change) with the difference being determined by the engine number, lower than 228389 and higher than 228390. I am not sure the difference between the parts.
(Look at item number 7)
Maybe a call to a decent dealer could answer any other questions. I doubt Triumph would ever admit why they changed parts mid-model because of quality issues. In the past others have said it takes about 4 hours for the dealer to replace.
Anyone else have any ideas? Maybe we could set up a poll and see how many have had a failure?