Well, I think we have an arguable conclusion here. I have examined all my M18 series helmets, fourteen in all, 6 with the original liner, and four each with the /40 and /63 pattern liner, and of those TEN sit absolutely flush on a flat surface, and FOUR have the uptilt to the rear. ALL those four have the Kyburz stamp. Three are with the /63 pattern liner, and one is a n early model with the original full-hoop liner band. I also have one other uptiled example which has no Kyburz stamp, but that is one of the comparatively rare Large size and the tilt may be as a result of the upscaling in size.
On this quite limited sample we could assume that the uptilt at the rear is an artifact of the Kyburz manufacturing process. Seems possible, but it would be useful to know how many other concerns made these helmets - and why they weren't ever given identification stamps! These people, they have no concern at all for the collectors who come along decades later.