The legend around Glock is that no one ever thought to make a demo cutaway model until one industrious machinist decided to create a couple in response to all the “Where is the damn safety!?” questions. By relieving certain sections of the frame and slide, you will destroy a perfectly good firearm, but you also get a peak inside the inner workings of the design.
The Glock was not “safety-less” it was just a simplified design. Three separate safeties are disengaged with simple action of pulling the trigger–and the factory cutaway makes these design features easy to see. Ghaston saw the first one-off design (legend has it that there were two) and liked it so much that he had them made as demo models for his sales people. A special serial prefix was set aside for the cutaway series so all but the first two will begin with the letters “JQ” (the two prototypes are AT prefix guns).
Over the near forty year history of the company, there have only been about 4,000 cutaway guns made across all models and all generations. There were only 90 Gen 1 model 17s made (or 92 with the prototypes). Gen 2 models included the models 17, 19, 20, 21, 22, & 23 but fewer than a thousand were made across all models. The Gen 3 series saw an increase in production and the same six models available, with the first Gen 4s showing up around JQ4000 (or so).
Factory cutaways are a unique subset of the collector market with a small following which can present some great values for the astute collector.