Locating the bounding box of a fortress structure

[Ben-Kantor]

Hi, I realize this is not the correct place to ask, but was hoping I could get some support or at least be told where to ask. I am trying to edit the bounding box of a nether fortress, but cannot find the BB tag that I belive should store 6 integers which dictate the bounding box of the structure. This picture shows the lack of the tag:

Artificially adding a BB tag with the modified bounding box does not seem to have an effect, is it now stored somewhere else? Or is there some other reason that the tag does not appear? I greatly appreciate any help.