It is possible that Hackl finished off the Lanc that Frass had damaged.
PB120 was a Lanc of 582 Sqn
Missing on mission to Cologne 23-DEC-1944
http://aviation-safety.net/wikibase/wiki.php?id=159043
PB137 was a Lanc of 15 Sqn lost
Missing on mission to Heinsberg 16-NOV-1944
https://aviation-safety.net/wikibase/wiki.php?id=159031