So as with the other thread regarding night lighting legality and approval that I just replied to you on, this is another issue where DAR's do different things. I HAD exactly the same interpretation of the rules as you do - "external configuration" includes dimensions - it can't just look like something, and on another forum, I told the OP that I didn't believe he was allowed to use the NX designation.
However, the OP was able to get a local DAR to tell him that using the NX designation was perfectly fine. Now, since he got what he wanted, he's not going to appeal that ruling, but by your (and my) interpretation of the rules, he SHOULDN'T have gotten permission to use the NX designation.
In this case, in the real world, it doesn't amount to a hill of beans, because there's exactly zero safety issue with not having an "experimental" lettering and having an "NX" designation, so who really cares, but it's just another example of DAR's "doing what they want", in a colloquial sense. In this case, to the OP's advantage, but sometimes, to a builder/flyer's disadvantage.