Very interesting thread :-)
I still don't get the intention of the marking. If the wheels were already on the car - why would a spare tire indication be necessary? I mean, if the line worker could see what rim was needed (black or color) he should be able to choose the right tire too? Maybe the correct size was a problem to spot at a glance, but the color of the stripe or RWL was so obvious... ?
what if the trunk was "loaded" before there were wheels/tires on the car
or the trunk was also the reference point for the tires on the car
