Depending on the design of the holder, the three screws are held to their mounting posts with a nut. On my current version or the Ilford MG filter holder the nut is first placed into the slot of the posts and then the screw is fed through from the outside. I think the holder shown has these these three screw & nut holding posts.
The dimensions of the post lugs prevent the nut from turning. Once the screw engages the nut, the nut is held captive between the posts. Then, rubber contact pads are installed onto the ends of the screws. These rubber contact pads serve two purposes:
1. They prevent marring of the outside of the lens barrel.
2. The rubber has significant friction with the lens barrel, giving a good grip with a moderate amount of pressure.
Since the holder and filter have so little weight, it doesn’t take much tightening of the three setscrews to obtain sufficient grip. The screws are snugged with the grip of your thumb and fingers only—don’t use a wrench.
I received a second used set with holder that was missing the rubber pads. I was able to find suitable replacement at my local hardware store. It’s flexible small diameter black rubber tubing sold as replacement automobile windshield washer tubing that carries washer solution from the reservoir to the spray nozzles. It’s also used to conduct water from the solenoid valve to the evaporator pad on my furnace humidifier. It’s a common product and readily available. It comes on a spool and is sold by the foot.
Cut three pieces with a razor or sharp knife. A scissors might work as well. Then you have to work the short rubber tubes onto the ends of the screws for a safe, non-marring grip on your lens.