Hexagon bar is a hexagonal (six-sided) cross section, size (Diameter) is specified by the distance across the flats.
12L14 Steel is a standard free cutting carbon steel, with smooth bright finishes after machining.

Material: Steel
Shape: Hexagon Bar
Alloy: 12L14
Temper: Annealed
Diameter: 0.375 (3/8 inch)
Finish: Cold Finished
Specifications: ASTM A108, UNS G12144
Tolerances: Diameter: (+0/-0.002) | Length: -0/+0.999)