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The levelcodes:

4: YAHOO 10: TRONIC 16: TEEACE
5: IMPER 11: DJAYBE 17: ZOZOR
6: BART 12: JAILE 18: PSNPSN
7: MBMBMB 13: GRIZZO 19: SILVER
8: PRINST 14: MOBYDK 20: KIKISF
9: EVERST 15: LOOPZ 21: WOOPER
51: BUSTER (Information from Cheat Machine)