Is there anything that ties that compass specifically to the Do 355? I very much doubt it, it seems an entirely bogus claim without some supporting provenance.
I would be amazed if the Luftwaffe had not used a standard instrument in its aircraft.
I don't have the hersteller code 'dxt' in my list, but it is in the range issued in April 1941, during a period when a new series was issued monthly (until September) so whatever the company was, it was manufacturing for the Wermacht from early 1941.
Edit: If anyone has 'dxt' I'd love to see it!