I'm not absolutely sure, but I think the placement is not always consistent. Seems to me that I have seen some later cars manufactured for different markets that still have the comm plate on the left side of the engine cowl. I think that therefore the door post placement was more for North American market. That would certainly explain why the rivet holes are still there after the plates were moved to the door post.
Thomason's book seems to confirm this assumption since all their photos (taken in the UK) show the plates on the cowl.
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