Copyright Shield Designs ?

[richsc]

however, if it is using common symbols of the period such as the cresent moons, thunderbolts, and other common symbols of the era

Isn't that called Prior Art?

I also applaud Legio XX, they share all, designs, methods of production and do the RAT community credit by their efforts.

You have Matt Amt's leadership to thank for that.
In fact, if someone copied our designs, and I think I've seen that around, it only means folks agree with us about the interpretation.
I think about the only thing you can really try to control is images of your faces, since that's pretty well understood. Even photos of your scabbards, helmets etc since they're all based on archaeological finds, are probably open.
Which is probably also why the film industry has original costuming since that can be copyrighted as intellectual property. The helmets in Gladiator cannot be reproduced without permission, for example.