To me that is the base plate. It's basically the same piece that we call a base plate on a non-adjustable. it just has the slots enlarged for the other piece that I call the adjuster plate.
Hey Stan, Did you notice I included "Butterfly Door" as an alternate name for "Silo Door" on the Version 3 of the Terminology photo/chart/guide? Thanks for that input!
Bed plate. I got out one of my Gillette adjustables to take a closer look and I'm standing by my answer of Bottom Plate for Part 1, but I'm deleting my comments.