As an engineer I'm feeling quite ignorant. How are these blades used. In the left photos, I see a "c"-shaped milled piece of steel with what appears to be a single blade across the mouth of the "c", forming a bow-like instrument. In the photos to the right it looks like a single blade on the "back" of the "c", leaving the mouth open.

Where does the film-base go? Does it go in between two pieces, or does the blade skim over the film-base?

I must say, I always visualized the blades as more squeegee-like. I saw the previous thread, but must have stopped reading it before the last several posts were added.