| http://www.w3.org/ns/prov#value | - i would say its a manufactured material made just for the purpose - could be acheived using, I can imagine, plexiglass/lexan - just sand one side, then print onto sticky transparency film what you want (ie, everything black except the numbers - use a laser printer or something) and finish with a black permanent marker or paint pen to make sure it is an even completely black finish except where you want
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