Dear Scott,
Over the course of a century of manufacturing, you are correct that two different materials were used. The porous-knit, mesh pouch is designed to regulate temperature. The solid pouch is designed so that the cup which is inside will not so easily tear the fabric in the course of movement.
There was a time that protective cups were even made out of metal, so you can imagine that the metal would tear the porous-knit, mesh pouch.
In my years as both an athlete and coach, it was a common practice in heavy contact sports (my background is in Lacrosse) that the athlete would strip down completely and strap into a regular, porous-knit jockstrap. Then, he would don a very light material (similar to today's boxer-brief) cotton short, and would next strap a protective cup over the short and the jockstrap underneath. He would then don his uniform shorts over the combination. It was tight, to be sure, but two forms of preventative sports medicine were being observed: the genitals were supported (literally) in order to minimize scrotal torsion and to provide temperature modulation using the regular jockstrap; the light material short was to prevent chafing and "raspberries" when sliding or skidding on the turf; and the protective cup was to minimize injury from direct impact to the area.
When sliding shorts and compression shorts were designed, they were also designed for a specific, preventative sports medicine purpose. The sliding shorts, as the name implies, are designed to minimize the scraping and injury to the skin when someone slides either into a base or on the turf. The compression shorts, as the name implies, are designed to compress the upper thigh region in order to minimize injury to the front and back sides (ham-strings, etc.). But! neither of these should ever be considered as an alternative to the athletic supporter, which has a different though important sports medicine purpose. Unfortunately, and inexplicably, many athletes today eschew wearing a jockstrap, and are under the impression that the sliding short or compression short (and even now, the "Peter Pan tights" which ostensibly straight athletes are wearing!) can replace the need for a proper supporter. Some companies have sewn pockets into the sliding/compression shorts, as an "all-in-one" for the athletes, but after several washings, the material in the sliding/compression shorts becomes less restrictive and the protective cup has the tendency to shift in the course of movement. What so many people do not realize is that when the cup shifts, if the athlete suddenly experiences impact, the mis-positioned cup can actually be wedged between the two testicles, thus causing horrific and painful damage.
Perhaps this is too much information, but your question is important and all of us must continue to emphasize, especially to the next generation of athletes, the literally vital importance of protecting the genitals by wearing a proper jockstrap and protective cup.
Let me know your thoughts.
Mike