If that RD is original, the b-screw should bear against the b-screw surface on the hanger. In the second photo, there's a wear spot on the integrated hanger, just where the screw should be touching it, but it's obviously not touching it. instead, the screw is pointing at the the piece of metal that's between the derailleur body and the bike's integrated hanger. Looking at photo#1, it kinda looks like that piece of metal is part of a broken-off claw hanger--the kind of bolt-on hanger that's use on bikes without integrated hangers (part #5 in SBinNYC's exploded-view diagram). It looks to me like this is not the original derailleur, though it could be the same model as was originally mounted (without the broken claw hanger). Assuming I'm right that that's a part of a broken bolt-on hanger, can you take it off? You should be able to do that, then screw it directly back into the integrated hanger, adjust the b-screw, etcetera.
Another bit of circumstantial evidence that this bike has been modified/repaired: I'd expect to see quick-release axles, rather than bolt-on.
If you're still scratching your head, then probably Tom Velo Orange's suggestion to take it to a bike shop is the best advice, though.
EDIT:
Tom Velo Orange mentions there might be a "key". Looking at photos 1 & 2 again, what looked to me at first in photo 2 like the broken-off top of a bolt-on hanger, might actually be such a key, rotated the wrong way.
If that's true, then this can't be the original derailleur. The original derailleur's b-screw obviously bore against the integrated hanger (what you call the "nub").
Hard to be sure, based on these photos, I guess.
Last edited by mcgreivey; 12-14-09 at 09:57 PM.