They are squadron markings, D in this instance with the individual vehicle number inside. The colour of the outline (a 'D' on its side. 'A' was a triangle, 'B' a square, 'C' a circle and HQ a diamond) should be white as the RCD were 'unaligned' within a brigade. If in a brigade AFV squadron outlines were red, blue and yellow depending on their precedence within the brigade.
Not sure about the background colour but black makes sense.
This from memory...
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