M3 and M4 nuts
Available with standard or reduced diameter heads. The reduced head versions are manufactured specially for holding M3 nuts when Neutrik and similar connectors are being fitted in densely packed panels, where there is often inadequate clearance to use a standard head tool.
Knurled nuts
For knurled and "castle" ("castellated") nuts with external diameter 8mm and internal 6mm, these have two lugs to engage (to some degree) with the knurled surface or with notches in the nut. They have a 2.5mm centre probe to held them centrally while tightening. Typical applications are some types of 3.5mm jack sockets.
Castle nuts
For "castle" ("castellated") nuts on connectors where the body extends forwards. These nut drivers resemble miniature "box spanners" and fit over the connector body, with two lugs to engage in the fixing nut. They have a 6.35mm hex drive fitting. 55-8195 suits DIN1.0/2.3 connectors. The measurements denote internal depth, external diameter and width of lugs.