Hex Set Screws - UNC
Hexagon set screws, or hex sets to give them their common name, are available in both metric and imperial thread sizes in a wide range of finishes. A set screw differs from a bolt in that the shank is threaded all the way from the tip to the head.
UNC stands for Unified National Coarse. It is a common imperial thread type with less threads per axial distance than its UNF equivalent.
Unified thread types (UNF and UNC) were introduced at the beginning of World War 2 to enable the interchangeability of machinery manufacturing between the USA, Canada and Great Britain. Still commonly found on a vast array of old machines and vehicles in the UK, UNF and UNC threads continue to be the standard thread in the USA and Canada.