Disney-comics digest #557.

[DAVID.A.GERSTEIN]

      P. S.  I believe that Dewey is indeed a real first name.  Also, 
no matter what anyone says, I imagine that "Dumbella" must be 
Donald's pet name for his sister from when they were kids.  A 
nickname for Della (not the other way around).  No one could really 
name their poor daughter Dumbella.  (Note to non-English speakers:  
a dumb-bell means a total idiot.)