I think this is a Carstensen conversion on the FL die.
$75 shipped
This is from a 2004 article by Carstensen on another site
The JLC Bushing Die Conversion
I've converted dies to accept sizing bushings for 10 years. I can take any die, either a neck or full length die, (with the exception of a carbide die) and convert it to resize the neck with a bushing. This allows you precise control over neck tension without the disadvantages of an expander ball. They work best with necks that have been outside neck-turned, so as to remove the inconsistencies that you have with virgin brass. Turned necks will also result in the bullet being centered better in the chamber. I think a majority of the growth that reloaders get in their brass is due to pulling the expander ball back out of a resized neck.
this takes standard Redding and RCBS bushings