The most recent published history of No.451 Squadron RAAF during WW2, "We Together - 451 and 453 Squadrons at War" by Adam Lunney, Tempest Books UK, published 2020, does not give any tie up between aircraft serials or individual aircraft id letters.
If the Squadron ORBs or Appendices do not record aircraft id letters against aircraft serials, then the path would normally be to try and get access to any surviving Squadron pilot's log books and hope that they recorded the aircraft id letters, not just serials. Again individual pilots, with different Squadrons would follow different practices in what they recorded in their log books so it can be very hit and miss.