Hex Bolts - UNC
A hexagon bolt, or hex bolt to give it its more common name, is a threaded bolt with a plain shank and hexagon shaped 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.