The D400 uses a metal aispirator body. The old version of the D400 (pre 1994ish) and all of the preveious D300 and D350 regulators had a built in metal orifice. This was superb as the metal orifce made the D400 very easy to tune and resulted in low inhalation efforts around .5" or .6" of water and the heat transfer was excellent and minimized dry mouth.
The new and "improved" aspirator used a plastic orifice that could be inserted inside the metal aspirator body. Compared to the older D400 it sucks as the plastic orifice is harder to tune does not (usually) result in as low a cracking effort and tends to insulate the gas passing through the aspirator which seems to increase dry mouth a bit. The upside was that if the orifice were damaged it was an $8 fix rather than a $45 fix. But on the other hand the metal orifce almost never got damaged and it did get damaged it was almost always because the tech damaged so it would not cost the customer anything anyway.
So if they bring back a "D400V", I'd prefer it have an all metal orifice.