md5 hash only first 512 bytes of file

You can use dd to pipe only the first 512 bytes to md5sum. However this will cause md5sum to be oblivious of the filename, so in addition replace - with the filename again.

find . -type f -exec sh -c "dd if={} bs=512 count=1 2>/dev/null | md5sum | sed s\|-\|{}\|" \; | sort -k 34;