Nice Mk II and IMO the remnants are hessian that has adhered itself to the finish of the shell and was not painted over you can see remnants of the liner from helmet stacking that are inbedded on the top of the helmet as well as a line on the side of the helmet that was very probably left from the liner of the same helmet that was stacked on top of it as well.
As to the liner retention nut and bolt being reversed James this is quite commonly seen and period done I have a good number of Canadian Mk II helmets in my collection were the nut and bolt has been reversed the reasoning is that the remains of the bolt that have been threaded through the dishpan nut can impact with ones head leaving a nasty mark and I can imagine one hell of a headache to go with it.

Regards Mark