All Gen1 and Gen2 Glocks, and the early Gen3 models (full-size frames), had a half-moon-shaped cutout at the bottom of the frontstrap, and another one on the rear edge of the magazine well. This would allow the user to pull a jammed magazine out of the weapon by force (using the forefinger and thumb, while holding in the mag release button), if the mag became stuck and would not eject normally due to a double-feed or other stoppage. I believe this frame cut-out was discontinued when the 3-pin frames were released; I have Glocks with both 2-pin and 3-pin frames, manufactured in consecutive months of the same year (2000), and the 2-pin frame has the cut-out, while the 3-pin frame does not.
It is probably unnecessary now that Glock's magazine bodies are slightly longer than they used to be. Nowadays, you can usually grab a Glock mag by the sides of the baseplate and pull it out, because it hangs out of the frame a bit farther than the old (shorter) mags used to, so there's more mag body available to grab, if needed.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)