OK, a couple of fine points to nitpick.
First, that indentation is the overlap of the PLIES (not the belts). It is indeed stronger there than the rest of the tire.
Why don't you see indentations in passenger car tires? You do, but a passenger car tire is inflated in the 30 psi range, not the 70 to 80 psi range. If you look carefully, you will find at least one indentation for every ply (not belt ply) a tire has.
Also, notice in the photo a circumferential indentation 360 degrees around the tire in the mid sidewall area - where the lettering is. That is where the ends of the plies are. The plies are turned up around the bead - a hoop of steel wires - and they end in the mid sidewall area. Because of differences in the amount of material, you get bulges and indentations, and that is one of those areas.
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CapriRacer
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