A rare pistol that failed commercially. Not much info is out there.
Markings include "F" & "S" on the left side of the reciever for the safety. "PATENT HAEUSSLER / ADLERWAFFENWERKE / ENGELBRECHT & WOLFF" on the right side.
One of these pistols in exquisite condition was auctioned at the James D. Julia Auction House, in October of 2015. It was sold for $6,750 USD.