This picture shows factory ported Glock pistols. The two most common are the C model pistols which you can see int he photo above as the one on the bottom. The other two are somewhat uncommon. The Glock on the top is a Gen 1 Glock 17L with an optional factory three port barrel. The middle Glock is a Gen 3 Glock 24C with 4 ported barrel. Somewhat similar to the Glock 18C porting like this one below.

Photo courtesy of DJ Niner from Glocktalk.com