I think it's a secondary place to put the jack without having to climb in and strap it in behind the spare. I could be wrong, but it fits!

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2003 530i, Silver on Grey, BC Coilovers, Square 9.5" M-Parallels, Gloss Black Roof
2007 328i, Alpine White on Black, Sport/Nav/Premium
SOLD: 2006 X5 4.8is, Lemans Blue on Black