I lean more towards the idea that the stamping is just very faint or totally missing. But if it it was masked I'd bet it was not done for the shots that later appeared in the chronicles. They don't look at all like promo shots but maybe made for continuity purposes that later were published because nothing else was available.
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Besides GRAFLEX is very visible on the clamp and even the markings on the transistors are to some extend, so what would be the purpose of covering just the wind vane for these shots.