It would be surprising if the officials in Reunion did not send measurements of the various dimensions along with the photos. Not to mention numerous gigabytes of high resolution close up photos.
Surely any engineer who has worked closely on a 777 flaperon can take one look at the photos and state if its origin is 777 or not?
How difficult can it be?
In an effort to exclude every other possibility, could the delay be due to Boeing trying to account for every flaperon they ever built?