I have as yet not fitted a metal clad 10 mA RCD socket and the idea was years ago when I got it to use it for garden tools, however the test button must connect line to earth vie a resistor not to neutral, so pressing test button where originally fitted it would trip the 100 mA and 300 mA RCD's feeding it, and the whole idea was should we have an item with earth leakage the 10 mA would trip first and not put out whole work shop, we likely had more trips due to some one pressing test button than we every saved by having discrimination. It was going in the bin when workshop was taken down so I grabbed it, and it has been in my box ever since.
As to special leads, I had a box of adaptor leads used to test with, from BA22d to 13A with croc clip on earth, to 63 amp to 13 amp they were only made so we could simply plug in the loop impedance tester never designed to be used for an appliance. However from time to time one would vanish, and this is the problem some one will at some time use them for other than what you intended, dim ddaear may mean no earth but using google to translate is not best option, there are countless times where it has gone wrong, at the local heritage railway we had to have a second sign made saying North Pole in Welsh as miss spelt.
And the point is we may know what "No Earth" means, but others may not, and just be careful not to let the cable drop on the ground. The general rule is if a socket has an earth or neutral it should be connected, if no neutral use 4 pin socket only and if no earth two pin socket only, labels are use less, consider where I want to charge my car battery, so I have a class II charger and looking at some distance house to car, so multi extension leads, want to keep it dry, so extension lead one goes to my Jazz and under bonnet the second extension lead is connected and runs to Sorento so if it rains no problem, if my wife then decides she want to entertain in the garden she could well simply take the lead from Sorento so if the first lead had no earth, she would have no idea of the danger of using class I appliances even if she was to understand what no earth means.
Labels with class II only, no ground, no earth or any other warning are only any good if the user knows what it means.