Here you go Mustafa, the US patent #528155 for the Dagor. Note the two constructions covered, the one on the right is known as the "reverse Dagor" - also used in the Zeiss Doppel Amatar and the Schneider-Kreuznach Angulon.

http://ip.com/pat/US528155 You have to click on the "download" box and enter a captcha code.