Bahco Robust F-shape Clamps with malleable cast iron brackets for strength and a galvanised steel slide rail for a smooth action.
They have plastic protection caps and virtually unbreakable handle grips.
420 is the style of clamp, second number relates to the throat and the third number is the maximum opening capacity.

This Bahco 420-100-200 F-Clamp has the following specification:

Capacity: 200mm (8in)
Jaw Length: 100mm (4in)