There you go, spend a few bucks and have by far the best mount of the three.Bottom scope mount is an SEI part. If the mounting bolt and cam is missing I think your can get replacement parts from Fulton Armory (see below). (Note: Brookfield mounts have a Weaver rail with only 5 or 6 slots for rings, as opposed to the Picatinny rail seen in the picture).
The missing D-clip is standard hardware available at, well any hardware store. The problem is as the mount is used the ring can rub the receiver finish off. I removed and have had no problems.I believe the SAI mount is a Third Generation. I am told they work fine if installed when new using factory instructions - which includes using a brass/soft hammer to form it to the receiver, and then kept with that receiver only! I notice the front knob is held in with tape - if the washer/e-clip is missing the end of the knob may stick into the left rail and interfere with bolt movement. I'm sure SA can furnish parts and instructions. If the one without the knob is a Brookfield or Smith, you have really scored! CC