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In the book photo I posted in your original thread ( https://desoto.org/community/general-discussion/new-member-from-thailand/#post-13734 ), it appears that the Diplomats have letters spaced out above the grill. If you look at the cars in the photos, they all appear to have them. We can't see the trunk to determine if they should be there too, but I'd say above the grill certainly seems like a good location. If your car at one time had them and had the holes filled, you might try looking on the backside of that panel to see if there are signs of the original holes or not.
The ones you have pictured look like the standard 1955/56 letters and we have extras for sale if you decide to add any to the back/trunk.
@LeeExline might have better photo documentation for reference.