I bought my first set of dies in 1967 , 38 special, RCBS , the Cardboard disk was in the box back then...I think it's still in there ! And now I know what it's purpose is , rust prevention !
At that time they sold 38 special and 357 magnum dies , the gunshop owner told me to put the 357 dies back and get 38 special . Then explained how to adjust them for 357 magnum. I found a brass spacer to use at the hardware store and thought I had a right smart idea there.
Gary