The full-size Glocks in 9mm/.40/.357 have always had smooth-faced triggers. Starting with the introduction of the compact (G19-size) handguns, compact (and later, sub-compact G26/G27/G33 guns) had a trigger with a grooved face. This feature was added to give the smaller models enough points under the "points system" import rules required by the Gun Control Act of 1968. To be allowed into the U.S., points are awarded based on features such as size, weight and caliber (generally, bigger is better), adjustable target sights, and grooved target-shooting-style triggers. Without the grooved trigger, the smaller Glocks could not be legally imported.
Glock trigger styles, smooth-faced on the left, grooved on the right:
After using Glocks with both smooth- and grooved-face triggers, many shooters find they prefer the feel of a smooth trigger; others just want the same style trigger in all their guns so they handle in a similar manner. Based on this, many of the compact/sub-compact guns' grooved triggers are replaced by users with smooth-faced trigger assemblies made for the larger full-size models, which are interchangeable. Although the replacement of the trigger assembly is not a difficult task, there are a series of safety checks which should be performed prior to using a Glock that has had the trigger (with attached trigger bar) replaced. If you don't know how to perform these safety checks, it is recommended that you have a gunsmith or Glock armorer do the replacement, to ensure safe operation and no unpleasant surprises during use.
Smooth and grooved trigger assemblies, including the attached trigger bar: