After discontinuing the ported Gen1 17L models, Glock didn't offer any ported models again until 1994. The new-for-1994 G24C long-slide was the same size as the 17L, but a little heavier (as were all .40 caliber versions of 9mm Glocks). It had a different arrangement and shape of ports on the barrel, but the ports were still centered on the barrel and still vented upward through the cutout in the top of the slide. The ports were also different sizes, with the first one (closest to the breech) being the smallest, the second being slightly larger, and the last two being the largest and equal in size.
G24C port shape and location:
The G24C was produced from 1994 to about 1998, although there are reports that occasional short production runs of non-ported (G24) models were released after this period. Although the G24C has been listed in the annual Glock catalog for the last several years, it appears actual production has been slow-to-nonexistent.
In the past, Glock also offered G24C slide/barrel assemblies for sale separately, so owners of service-length G22 .40-caliber pistols could "upgrade" to a target-style top end to make their Glocks more versatile. Ads for these slide/barrel assemblies can be seen in some of the late 1990s "Glock Annual" or "Glock Autopistol" magazines/catalogs.
EDIT: The 24C shown above is a Gen3 model. Some early Gen2 versions of the ported G24 were not marked "24C" on the slide; they were marked the same as the unported models ("24"), and the box label had a "-P" after the model number, designating a ported barrel.