I think I remember what you're talking about but I haven't seen one in quite awhile.
As I recall, they were installed by a distributor and were more or less permanent once installed. That gave rise to the rumor that the pistols came from Glock that way.
They were very small, fitting entirely on the front of the trigger guard with space above and below to spare and protruding forward maybe 1/2" to 3/4" from the triggerguard.
I believe that LaserGrips used to (maybe they still do--I don't know) market a model called the GLS-630 that was something like what you describe as well.