I was at the dealership today switching over to summer wheels/tires and while there asked my SA about "in service date". He said it is date vehicle is first sold. He had never heard of it every being based on any kind of production date. So I think that info is just wrong.

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2012 135i Coupe,Jet Black, MSport, 6MT, Prem, Exec,Nav
Gone: 2012 135i Coupe,LeMans Blue, MSport, DCT, Prem
Gone: 2011 X35i, Vermillion Red, MSport.
Gone: 2007 328xiTouring, Barbera Red, Lemon, 6MT.
Gone: 2009 M3 Sedan, Alpine White, Black Novillo, M-DCT.
Gone: 2008 535xi, Alpine White, Black Dakota, MSport.
Long Gone: 11 other BMWs between 1971 and 1986.