I have a early model "B" ser # 18XXX with an unusual screw off threaded piece at the end of the barrel. Pretty sure it was for a silencer. I know a little about the history of the selection used to choose pistols used with silencers but cannot find out info for this one. I know the government settled for the HD. What can you tell me. Could this pistol be a proto type used in the beginning stage during WWII?