These are screws with a standard metric (M) thread with a wide flat knurled head. They have a base.
This is an all-plastic Bülte-exclusive screw. You can find in the plastometallic screws subfamily, its bi-material derivative in black nylon/metal.
All these screws are available in a wide range of colors on request and they also exist without a base.
Thumbscrews are often used where there are fine adjustments or where frequent loosening of the screw is required.