Do a windows search for druck_radio and see what comes up, it might be as simple as druck_radio_type21.tga.:hmm: Or if you're unlucky, these files are packed in the relevent .dat file.
The XXI was never really
finished, so there is a lot left wanting.
As far as skins go, IIRC Fubar is the only person I know of that has skinned this boat(edit - maybe Venatore aswell). The XXI is a PITA because it can only be skinned by unpacking/repacking the dat file. It's a lot of work for a small gain.(considering the amount of time you actually get to use this boat)
I can't check files at the moment but I'm sure that if you investigate the Type21 dat file you should find what you need. If you are thinking about skining the 21 then you should know that there is no separate conning tower dat, it's all one model/graphics file.(which is why the 21 is the only tower to 'show' damage when hit.
)