I think you're referring to the Resizing Ring. If so, that 400 used to have one, maybe got lost? Might not need it if the shells are used in the same gun they were originally fired in. Shells with steel "brass" might be a bigger problem than shells with real brass.
What crimp do you plan on using, folded or rolled? If you're going to star-crimp, you might be in trouble. Neither the Crimp Sleeve nor the Crimp Die will work with a 28 Gauge hull and MEC doesn't have parts for the 400. Also, the 400 doesn't have a Crimp Starter, making for poor crimping with plastic shells. If you have a Roll Crimper, that would solve those problems.
DC