The Schwinn GT500 was Schwinn’s version of an updated Huret Allvit, supposedly ready to take on the might of a rampant Shimano and an all-conquering SunTour. This was never going to be an easy task, but even so the GT500 was a pretty feeble effort.
The body seems to swing further forward at the unsprung pivot, the sprung pivot has been moved back, and the pulley cage has been lengthened. These all look like fairly sane moves to improve gear changing over a reasonably wide (14 to 28 tooth) freewheel.
The new feature, that is insane, is the massive flange on the pulley cage. This is allegedly a safety feature to allow the rider to back pedal in the wrong gear when stationary - or something. We are not told whether they have to be smoking a pipe, balancing a monkey on their head or wiggling their ears at the same time. The Huret version of this derailleur was called the Allvit Safety in honour of this uniquely useless attribute.