The term master setup is generally used by American Animation
cel dealers.  Which just means that the background is the
matching one for the cel.  

In American Animation background are very frequently sold
seperately and often cost more than the cels!  So you can
have a cel w/background setup which will not have the
matching background but still cost more (since the
background are expensive, sometimes up to $5000 a piece or 
more) and you can have the master backgroud setup which
is like what was metioned earlier that it is the correct
cel for that background.  Older background, expecially
disney are very intriticate and extremely detailed.  Just 
look at some of the scenes from Snow White, Sleeping Beauty,
Pinocchio etc and you can see why they cost more (although
$5000 for a background is a lot of $$ any way you try to
justify it).