Our intrepid spy photographer was quick enough to grab decent shots of the lettering on the tires. What they reveal is that this Shelby is rocking 20-inch wheels up front with 305-mm tires. Those familiar with the GT350 will know it uses 19-inch wheels, both in standard and GT350R trim. If you look up tire sizes for both models, you'll also find that the normal GT350 uses smaller 295-mm tires up front, and the GT350R has 305-mm tires. We don't think this is a just special GT350R either, since the wing is absent, as is the extra-deep front splitter.
Aside from the tires, there isn't much more we can discern from the photos. What we can see through the camouflage seems to show roughly the same front and rear bumpers as the current GT350. However, there are probably small tweaks to them to put the styling in line with the refreshed 2018 Mustang. As for when the GT500 will make its debut, we would expect no sooner than the next Detroit auto show.