Because of the unconventional 4 30 date window placement in the classic El Primero movement (now known as Cal. 400), Zenith created a new calibre, Cal. 400B, which moved the date to the 6 00 position. This modification required much reengineering of the movement, resulting in a jump in component count from 278 to 328.
Cal. 400B was also produced in titanium.