I was talking Hasbro produced action figures. That seems to be the main source for custom fodder in this scale. The head in the picture is from a G.I. Joe 25th Anniversary Ace.
There's been a bit of scale creep in the action figure market. Lines that used to be 3.75" are now 4" or slightly bigger.
I don't have a set of calipers, but something closer to 1.05 cm would open the number of action heads this helmet would fit. I based this on some heads sculpted by Boss Fight Studios. They sculpting a lot of the recent 4" Kickstarter action figure projects.