https://www.kmshooting.com/catalog/c..._complete.html
They are expand irons like those made by K-M in the link above.
When your turning necks on your precision brass so it fits your chamber concentrically and not pushed over to one side like most brass these are used to make sure the necks fit onto the turning mandrel without excessive clearance.
On wildcat cartridges they are used to create a false shoulder in the parent case so the base of the case doesn't move forward into the chamber from the firing pin strike.By holding the case firmly in place you avoid thinning the brass right in front of the web area but you can still move the shoulder forward and 1/8 inch without the headspace issues you read about here daily.