I'm aiming for 3-4mm.
I think due to the moulds the helmet tends to naturally thicken at several points with the visor being one of them because of the way the material builds up by the depression of the visor. The keyslots is another. Looking at the uniform thinness and tidyness of the production helmets makes me think the keyslots might be a totally seperate piece added in. I couldn't say for sure. A picture of the inside of the PP2 plays into that theory a bit but I could be utterly wrong so someone with more expertise would be needed to comment.