Delete/move files which in all respects have same file name except one set has a specific prefix added - bash

Question

I have a large set of working files of the form a.mp4 b.txt c.avi d.doc etc (the extension is irrelevant to the question). I also have a set of files which include the same named files except with a common specific prefix "broken_" eg broken_a.mp4, broken_b.txt
If I have a.mp4 and broken_a.mp4, I want to move the broken_a.mp4 to a holding directory. If I have broken_d.mp4 but no matching d.mp4, then leave it alone.
I have some code successfully used to identify and move files with the same extension which I'd like to modify
This is the form of working example code for same extension files (kudos to the original author) which I'd like to modify if possible to do the job
#!/bin/bash
# Name of source directory
SOURCE_DIR=.
# Name of destination directory
DEST_DIR=already_converted_m4v
# Create the destination directory for the moved files, if it doesn't already exist.
[ ! -d $DEST_DIR ] && mkdir -p $DEST_DIR
find $SOURCE_DIR -maxdepth 1 -type f -iname "*.avi" | while read fin
do
#echo "m4v doing avi"
fm4v=${fin/.avi/.m4v}
[ -f "$fm4v" ] && gmv -v --backup=numbered "$fin" $DEST_DIR/
done
My garbage first attempt which clearly doesnt work looks horribly like:
#!/bin/bash
# Name of source directory
SOURCE_DIR=.
# Name of destination directory
DEST_DIR=Already_broken
# Create the destination directory for the moved files, if it doesn't already exist.
[ ! -d $DEST_DIR ] && mkdir -p $DEST_DIR
find $SOURCE_DIR -maxdepth 1 -type f -iname "*" | while read fin
do
#echo "working to find existing broken and unbroken files"
filetest_basename=$(basename "$fin" )
filetest_extension=$(extension "$fin" )
echo $filetest_basename
echo $filetest_extension
fileok=${filetest_basename/!broken_/broken_}
[ -f "$fileok" ] && gmv -v --backup=numbered "$fin" $DEST_DIR/
done
Grateful for help

Answer 1

find is irrelevant here, a simple shell loop would suffice:
SRCDIR='.'
DSTDIR='Already_broken'
if ! [ -d "$DSTDIR" ]; then
mkdir -p -- "$DSTDIR"
fi
for broken in "$SRCDIR"/broken_*; do
if [ -f "${broken%"${broken##*/}"}${broken##*/broken_}" ]; then
echo gmv -v --backup=numbered "$broken" -- "$DSTDIR"
fi
done
If its output looks good, remove echo.
Some notes:
We can't use ${broken/broken_} here for SRCDIR might contain broken_ in the future.
Nested PE (${broken##*/}) needs to be quoted for its result might contain metacharacters and that would bring about undesired results.