It seems that photos of 4343s with grilles intact are pretty hard to come by. Worse, it seems that some grilles, like mine, have these plates in front of the tweeters. Unless JBL were doing some very sophisticated sound-engineering I doubt if this was intended. I am still searching for an authoritative photo of JBL originals. If anyone has such a thing I shoudl be very grateful to receive it. I would then try to get the exact positioning and publish the figures.
I would never have believed such a "simple" question would be so complicated