I have found that there are dull and shiny helmets on each date stamp. There does seem to be some variation in the dullness or the shine.
Date Stamp
12041 this stamp has the helmets that have the most shine and the dullest of what i have found.
11971 & 11921 have both the shiny version and the dull version, but not to the extent of the 12041 date stamp.
All of this may just be my imagination, and I'm still sticking with the "two version" theory, but i would like to know what you have found.
It's hard to get a good pic while they are in the bubble, but i am attaching a pic of the difference between the shine and the dull.