Hello, Thomas. This is an interesting question. Apparently there were a number of boats operating in the general area of USS Guadalcanal at the time of U-68's sinking.

The Allied Intelligence report on the sinking of U-68 does not seem to indicate any unusual questioning of the survivor (see [www.uboatarchive.net] ), although it must be noted the large groups of survivors seemed more likely to "band together" and be more disciplined about security while sole (or groups of two or three) survivors appear to have resistance to questioning overcome more easily. Kastrup does appear to have been free with his information. What was interesting about his questioning?

BTW, the above-linked report reports the survivor as being "Kastrup, Hans, Matrosengefreiter 2nd class, age 19".