Why "definitely"? Because they are claw mounts?
I believe you could order claw mounts installed at the factory on your Brno Model 21.
I have seen many of these on Brno rifles and they all look exactly the same, and all avoid covering
the CZ trademark on top of the receiver ring, unlike the variety in claw mounts installed by
independent gunsmiths. It really doesn't matter much to me whether or not it was installed
at the factory, but I do believe it could be ordered. Do you have
evidence that this was not the case?