A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Wolfgang Köberer
Date: 2018 May 24, 10:54 -0700
The "DHI" stands for "Deutsches Hydrographisches Institut", the predecessor of the "Bundesamt für Hydrographie" which tested sextants. The rest I can only guess:
it might have been tested in 1978. You could try to contact them: http://www.bsh.de/en/index.jsp.