Two reasons - first, because the size of the aiming circle could (on many examples) be adjusted to match the wingspan of the target so the pilot would know when he was at the harmonisation range.
Second (and much more important) because the aiming mark remained on target regardless of the position of the pilot's head: if he moved his head too far out of line, the aiming mark would disappear off to one side. With a painted aiming mark, you would have to be certain that your head was in
exactly the right position, otherwise the sights would be wrong.
Tony Williams: Military gun and ammunition
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