I know exactly what that is! It's the "Five Wide Virgins," an unsuccessful predecessor of squid.
The same idea - fire something ahead (actually standard depth charges, pre-set to some hoped-for depth) while still in contact with deep S/M. Actual name was a Thorneycroft Mortar, fitted in HMS Whitehall in 1941.
The more successful Hedgehog of later in 1941 was trialed in Westcott and was used quite widely until squid began to appear by late '43. Another was Parsnip; that was like Hedgehog, firing 20 bombs, but taking up more space with 2 x 10 rows of mortar type tubes. Not developed.
All this from Willem Hackmann's HMSO "Seek & Strike - Sonar, Anti-submarine Warfare and the Royal Navy 1914 -54" (1984). A most valuable book! He's also my reference (among others) saying ASDIC had nothing to do with any nonexistent Anti Submarine Detection Investigation Committee of the early 1930's. While some underling at the Admiralty told Churchill that when he asked for an instant response to a House query, Asdic as a term was in use by 1917 - 1918 (I have official reports using it) and stood for a device developed by the Admiralty's own Anti-Submarine Division + '-ics', like orthopedics.