These are the original aluminum thumbscrews supplied by Omega for decades for mounting lens discs and lens plates on a number of Omega enlargers with interchangeable lensboards, including the D-5, D-6, C67, B66, B22, etc. These screws are also used on the Omega B-22 to secure the lamp cap to the condenser housing. Omega no longer supplies this screw, having replaced it some years ago with a steel or brass substitute that has a smaller knurled area. We much prefer the original.
The screws are sold in pairs. A heavy-duty stainless steel version of this screw is also available - please see the separate listing.