two m40 helmets.

[Wesley's Dad]

In my opinion these are simple tooling differences. I am of the opinion that this difference is of no significance and cannot even be attributed to one factory versus the other. Your helmets versus Dom's would seem to confirm that. Another possibility is that the rounded corner could be a result of wear on the forging die the helmets were stamped on.

[maximwolf]

just different years of manufacturing. that is why different liners inside.