I just investigated the source of some objects (like the MP) being invisible in some menus (like the inventory).
And I rather quickly found the menutransform definitions in models.ufo. It seems to make these items visible again one could add appropriate menutransform definition. I'd do that, but I don't realy know how to find the proper values.
I also tried to find out why only some items need those definitions. So I compared the pistol with pistol2(MP) and guess what, I couldn't find a difference, even switching their models, didn't changed that, pistol showed the new model and MP was still invisible.
So at the moment it seems the only thing that makes the MP invisible seems to be the fact that you call it an MP...
Has anyone more info about this bug? It's realy annoying because you can't use the invisible items.