The original EF 28-105 f/3.5-4.5 USM (introduced 1992) had 5-blade diaphram. At some point (1999?) this was changed to a 7-blade diaphram. There were no other changes. I don' know the before/after serial number break. I have the older 5-blade design and have been very pleased with it and recommend it without reservation, but one might expect marginally better bokeh with the latter design.

The "II" version introduced in 2000 is entirely a cosmetic update. No change was made to the optics. Per the description at the Canon Camera Museum(http://www.canon.com/camera-museum/index.html), "This lightweight, compact standard zoom lens is practically identical to the EF28-105mm f/3.5-4.5 USM optically and mechanically, but it has a leather toned exterior finish for a higher quality look."