Found a little bit about the Depth Charge Direction Indicator on Google, but not a whole lot.
http://www.maritime.org/tech/radiocat/dcdi.htm
http://www.maritime.org/wish/dcdi.htm
https://www.google.com/patents/US2680238 (US Patent from 1946, so not the version actually used late in the war)
https://archive.hnsa.org/doc/ecat/cat-1419.htm