42MM Ballon Bleu WSBB0027 (2025) Pre-Owned CartierThe Cartier Ballon Bleu de Cartier reference WSBB0027 is a 42mm stainless steel watch from the Ballon Bleu collection, introduced in 2007 and defined by its rounded, double curved case profile. The Ballon Bleu 33mm (Ref. W6920071) is famously known as the "Kate Middleton" model and remains highly sought after for its timeless steel design. For high end collectors, however, the most desirable pieces are complex references like the Moonphase (Ref.