I found some pictures in the Maciejowski Bible where you see it and a palaeopathologist Dr Alan Ogden saying about the Towton mass graves that some "had part of their head sliced off, or their head cut in half" http://news.bbc.co.uk/local/york/hi/people_and_places/history/newsid_8673000/8673322.stm
But my question is if there are contemporary sources on swords actually cutting through helmets. I myself am just an amateur so i wouldn't know where to look. Thanks!
It's a bit earlier than my time period, but my understanding of the Towton skeletons is that they were generally men (and not a few boys) who were given fatal wounds after being incapacitated. They seem to have either removed their helmets while fleeing, or have been stripped of their helmets and then hacked to death, or hacked to death from the face (the unarmored front of the helmet), or a combination of all three. This book may be of interest to you.