No personal experience though I seen a bit of an endorsement by a YouTube video. Look at 28 min in if you like. Bottom line Id gather the ID of the die may be nitrideded or other coating to improve wear resistance of the die. I have not done any serious research for quite some time now on this die. I am very happy with the Whidneer dies I have been using for roughly 18 months now.
https://www.youtube.com/watch?v=ZpD7CdA-V9w
https://www.youtube.com/watch?v=ZpD7CdA-V9w