Hi,
Just to add a bit of fuel to the debate;
I've just had a look at the 1960s MK plug I mentioned above (no.48) which has the spring loaded sleeves and it does NOT have a hole at the top. Just a notch to locate the cover. Also, I had a look at some WG plugs from the 1970s, (available in various colours from Woolworth's) and they don't have a hole either!
So. . .
Cheers, Pete