^^^^^^ Just like with motor vehicles, model year changes start sometime late in the previous year. I forget the particular SN scheme in this case, but if L means the 12th fortnight (sounds like a top-ten rejected book title idea, but I digress...), then yes, that's fairly "early" (early June) in '87 to expect to see an '88 roll off the line (so to speak). But if it's the 12th month, then it's perfectly sensible that a December '87 build would be for an '88 bike.