The Canon EF 24mm f/1.4L II USM was introduced in the lead-up to Photokina 2008, as an update to its pre-existing 24mm f/1.4. Introduced in December 1997, the previous version of the 24mm f/1.4 was probably getting a little bit long in the tooth, despite having all the fixtures and trimmings of an L-class USM lens.
The upgrade to the lens is fairly substantial; it features 8 diaphragm blades in the aperture instead of 7, and the blades are now rounded. The lens elements have been regrouped, and an additional group is now present. Two ultra-low dispersion (UD) glass elements are used instead of just one. Finally, the new version of this lens is slightly heavier (100 grams) and a bit longer and wider. Read the very good review here. You can buy this lens here