I would guess that what you have found that the caliber list ends with the 5.6x52R is correct. There is absolutely no way that the action in question could have a 12 ga. bbl fitted. (.410, perhaps, but that would be all.) The action is way too petite to have anything very large on it.
Here is something interesting. I measured the mono-block diameter of a Brno ZH204 in 12 ga. That diameter is
27 mm. I also measured the mono-block diameter of a ZBK110. That measurement is also
27 mm. So, based on these measurements, it is indeed possible to mount a 12 ga. barrel on a ZBK110 frame.
