The aluminum "feed/bleed" piston uses a thinner pressure plate than the steel piston variety. If you don't have one, you can omit the wave plate and swap steel thicknesses around as long as you get your stack up close to where it should be. The billet piston exists because the factory aluminum piston is failure prone. If you do use a wave plate you can actually tell a difference in the way it air checks depending upon how you orient the wave plate (whether you put the "down" areas of the wave directly against the holes or not).
The forward drums that came from the factory with the feed/bleed setup don't have the bleeder ball capsule in the drum itself. I've never put a steel piston in a feed/bleed drum but I've heard it won't hurt anything even though it has no way to purge air from the circuit.