Red mark on tires
I'm almost certain that I've read that the red circle was to identify tires (of all sorts, solid and pneumatic) made with synthetic rubber that was believed at the time to be inferior to natural rubber, particularly in resilience and resistance to chunking when used on tank bogies. Looking at the toughness of some of the original tires you could easily believe it, but there were also period tires that have had the cords exposed and so rotten they can be torn by hand. Modern synthetics have wonderful qualities of wear, traction etc. but I wonder what they'll look like in 60 years. You win some, lose some.
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