The Glock design retains the pin using the slide stop spring. The pin is rebated in the middle and slightly larger at both ends. The end of the slide stop spring bears against the locking block pin and is supposed to retain it. It does keep it from walking out to the right (viewed by the shooter with the gun in firing position), but the pin can move a long way to the left before the spring catches on the right edge of the rebated section and stops the movement.
Stock Pin
There are aftermarket locking block pins that have grooves on each end of the pin in the appropriate spot for the spring end to "lock into". They should prevent motion in either direction.
The only one I know of off the top of my head with this feature comes in the ZEV titanium pin set. There may be others.
ZEV Set from Glockstore.com (Middle pin in picture is the locking block pin.)
