Sorry, they look nothing like any genuine
Roman military diploma fragments I have ever seen.
The form of the letters is way off and it looks, from the photos, like the letters are raised - if that is the case then it is absolutely certain that they are a modern
cast fake.
If the dealer is truly a reputable one they should take them back.
It might be worth your while grabbing a bunch of images of
military diplomas from online, especially ones from museums, but also from higher end dealers. You will quickly notice that they all look quite similar, and nothing like yours.
You could send the images along with your request for a refund and then you are not justing quoting some "so-called online experts", even if we are in fact
good ones.
SC