If this CNC machine of yours is a non portable  thing then wouldn't it be easier to power it from a 20 or 25 amp double pole isolator? That would get round all the problems of which type of plug is best and you wouldn't lose any portability if it a big heavy lump if a machine. Also south African 15 amp plugs are now rated at 16 amp although the design is the same as when it was 15 amp.  I always thought those old plugs looked capable of carrying much  more than 15 amp the plug pins are very beefy 

the plug pins are very beefy 

Although it's usually the springy socket contacts rather than the c.s.a. of the pins that's the limiting factor ... it can be quite a challenge to get a decent reliable contact area, especially on a curved surface (and one where the radius of the curve may reduce slightly over time as the pins wear) - one of the design features of the 13A system was flat pins is that the flat contact areas provided better and more reliable contact.

   - Andy.

Back in the 1940s flat contact areas with spring force were considered superior because it's easier to guarantee maximum contact surface.

But the thing about electical contacts is it's not really about contact surface - look at similarly rated switch contacts and they're much smaller. The primary concern is a thermal one.

Turned pins into a metal hole with a transition fit gives a very reliable contact, and if it's past it's best it's got enough copper to draw away heat from hot contact spots.

A spring brass contact onto a flat pin when overheated tends to anneal which then results in permanent reduction in performance.

Back in the 40s it would have been expensive to make round contacts with the precision that we now get in a £10 commando socket, and good engineering balances cost with benefit.

I wonder if maybe when the european CEE socket standards were adapted in the 1950s & 60s, and across Europe we had perfected the manufacturer of precision turned brass for WWII ammunition, maybe the economics had changed.

Keir Stitt said:

Turned pins into a metal hole with a transition fit gives a very reliable contact, and if it's past it's best it's got enough copper to draw away heat from hot contact spots.

Not all plug pins are solid brass ... I agree that most industrial plugs and appliance inlets to BS EN IEC 60309 series tend to be ... but provided the temperature rise, durability, etc., tests of the standard are met, I don't believe there would be anything to prevent a product with a hollow, or plastic filled hollow, contact pin (give or take getting patent licenses where necessary, as with other design features).

Keir Stitt said:

Turned pins into a metal hole with a transition fit gives a very reliable contact, and if it's past it's best it's got enough copper to draw away heat from hot contact spots.

Not all plug pins are solid brass ... I agree that most industrial plugs and appliance inlets to BS EN IEC 60309 series tend to be ... but provided the temperature rise, durability, etc., tests of the standard are met, I don't believe there would be anything to prevent a product with a hollow, or plastic filled hollow, contact pin (give or take getting patent licenses where necessary, as with other design features).