Voodoo is correct in that the 10th digit is the year digit, so we are currently on 'J' for 2018 and shortly moving to 'K'
The key points are when we went from F to G was the transition from mark 1 to mark 2 (although by that stage many of the updates to the engine had been made) and as we went in to 'H' was when we swapped over from euro 3 specification to euro 4 specification.