Just use bash (I'm sorry, Richard! well, yes, bash or whatnot- but.. the real star here is;) Use gnu findutils

for dir in $(find /rootdir/101/ -type d -name "B??"); 
do find $dir -type f -exec rm '{}' \; ;
done;

# or to move elsewhere
# do find $dir -type f -exec mv '{}' /tmp/foundstuff/ \; ;
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If you know to expect the filesystem hierarchy you suggested- There's not even a need to play with the B?? dirs..

$ find /rootdir/101/ -type f -exec mv '{}' /tmp/foundstuff/ \;
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