Thread
Snapz Pro 2.0.1 should do the trick. I'm not sure where you'd find an older version or if Ambrosia would still be able to register it (they should be able to; I registered 12 year old shareware ten years ago) but it would be worth checking out since it exports to QuickTime to create movies. It requires color, not sure of other processor requirements, but it does work with System 7 from what I've heard.
Another possibility is that screen captures, especially video screen captures, were created using a Mac that can output its display to a television, such as a machine using the Apple TV adapter (a Focus adapter rebranded) or a Mac with onboard video output (like the 660av/840av) hooked up to a video recorder of some sort. I screen-grabbed my pismo this way once. (using the video output hooked up to a Canon GL2 DV camera.)
Hmm?