Kelsey-Hayes proportioning valve for A body

Not to sidetrack, but I have one exactly as in the picture. What is the piece at the top circled in red? Thanks.
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I think the name of that fitting is called a Residual Pressure Valve. It's what designates the whole shebang as being for front disc brakes. The whole brass part below it is divided into two halves. Top part is above the mount bracket, with 1 line into the part you circled and 2 lines out going to the front disc brakes. Bottom part has 1 line in and then 1 line out going to the rear drums. Unless I misunderstand them, the only difference between the disc and drum blocks are the valve on top. I have a diagram drawn on a piece of cardboard and stuck up in the shop showing where each line goes because my CRS makes me forget! :realcrazy: