Older Glocks from Gen1, Gen2, and perhaps some Gen3 models had two other methods of front sight attachment, besides screw/bolt on.
One method was staking and an Armorer could buy the tool shown in the attached photo. This tool would flare out the base of the front sight once that front sight was inserted into the front-sight-hole in the slide.
The other method was for a sight designed to have a small, polymer pin inserted from the bottom, also resulting in flaring out the front sight base. This pinning method negated the need for the staking tool and gave some degree of being able to re-use the front sight (if the pin wasn't lost).
(Sorry, no pics of the different front sights, but are likely available during a search which I'll attempt once I spot typing.)
The staked-on front sights may have come in polymer or metal. Coming from the factory, night sights were typically metal, but also staked on. Aftermarket sights, whether by Glock or other brands, typically come with screws/bolts as they do now.
Here's a pic on found doing a search. As it's not my photo, I'll just post the link:
http://i38.tinypic.com/zly92u.jpg
A staked on front sight could be removed by punching out from the bottom; or taking pliers, protecting the top of the slide, and twisting off the front sight. A staked-on front sight wasn't intended to be reused, but with the staking tool, a person might get by with reusing a front sight which had already been used once.