Take it straight from the horse's mouth: Bruce Owen who was Purdey's production managr. In an article in Shooting Sportsman about 12 years ago he said the following-

-CNC machinery has brought increased precision in parts

-CNC machines also mandate the use of better steels because the formerly used mild steel cannot take the forces generated during machining.

Presumably certified steel also takes more prcise and controled heat treatment. Additionally there are modern quality control methods like X Rays, magna fluxing etc to ensure that there are no internal structural faults in the metal.

Pretty much the same is said by Holland and Holland managers on video. Modern guns are made of better material and with more precision.

Beyond England, the late Giovanny Metelli, chief of the Beretta custom department, told me that the later SO series shotguns incorportated improvements in design, material, and precision over the early models made in the 1940s.

My confusion is with the prewar term "best" gun. If they were indeed best, the new guns made of certified steel are "bester"?