Hi Samir, like Mark wrote, lettered cylinders have been around since 2011. Prior to that they were numbered by year, so a “9” stamped on the cylinder indicated a 2009 production year. Every few years Rotax mandates that certain parts will be phased out. Last year, for Micro and Mini, 2010 cylinders and older were no longer legal. Many good 2009 cylinders were taken out of circulation, so everyone began hunting for good lettered cylinders. As far as Junior and Senior, the original plan was to ban numbered cylinders this year, but although it’s not been made official, I’ve been told that numbered cylinders are going to be allowed again for 2016 for Junior and Senior. The latest batches of cylinders from 2015 have proven to be quite even performance-wise so far, the problem is they aren’t as good as lettered cylinders from 2011 and 2012.