Mounting Shimano Acera Derailleur: Supposed Gap? When installing a new Shimano Acera derailleur instead of an old, broken one (same model), I noticed that there is a little "opening" (marked in red) in the "frame" around the bolt that connects the derailleur to the bike, and it seems to be the negative shape of the bike frame (also in red, second picture). 

first picture:


second picture:


However, I cannot get them aligned, due to the parts marked with yellow. If they were aligned, it seems that I could just screw the whole derailleur a tad closer to the bike, and this gap would give more stability with respect to rotation, which would seem reasonable to me. 

I found no instructions for this problem online (except an explanation of the purpose of that screw by the yellow-marked area), but I want to be sure that I am not getting on my bike with a screw not as fastened as it should be.
I think the yellow landing is for the B tension screw to press against.  That will adjust how hard the derailleur pushes away from the cassette, essentially adjusting the gap when its at the big cog on the cassette.

Small chance that your new derailleur is for a different style of hanger ?

Do you still have the old derailleur to compare?