Okay, let me try to sort through this. If I've made any errors, please feel free to point them out. I've tried many times to sort out how the Beyer lineups work, and I definitely wouldn't be surprised if I've gotten something wrong.
The DT990 Pro, last I looked, uses the black plastic grill, similar to its original appearance since it was introduced in 1985. The DT770 Pro, similarly, resembles its classic 1985 appearance. Neither resembles their Premium counterparts, which contain more metal in their construction (though, of course, the Premium DT770 is no longer in production).
The DT880 Pro looks very similar to the DT880 Premium, however. This is likely because, unlike the other two, the DT880 of today is not a continuation of its historic counterpart, which was released in 1980 and discontinued and replaced by the DT990 in 1985. The modern DT880 is a reboot, first released in 2003 in order to create a more neutral headphone to recall the basic tonality of the original DT880. The original (1980) DT880 looked kind of similar to the original DT990, except it had a metal vents instead of plastic, which they probably figured wasn't visually distinct enough.
In 2005, Beyer revamped the aesthetics of all the Premium models, offered them all in three different impedances, and established the Pro/Premium dichotomy for all three lines. Before that, I don't believe there was a DT880 Pro based on the 2003 model. I'm not entirely sure that prior to 2005 the DT990 and DT770 existed as distinct Pro and Premium models, either, though here I could be mistaken.
Somewhere along the line a special 80 ohm version of the DT770 was made exclusively for Guitar Center, and if I remember correctly it resembles the DT770 Pro aesthetics. So there's yet another model out there.
Isn't this confusing?!