Re: What is this
Pretty sure that's the hold-off valve. Basically prevents fluid flow to the front brakes until some pressure threshold is reached. This lets the rear (drum) brakes take up the slack in the system and apply slightly before the fronts start to do any work. Net effect is (allegedly) a stabilizing effect (since the rear brakes will tend to straighten out the car when applied LIGHTLY) and reduced front brake pad wear (presumably by increasing the rear drum wear a bit and letting the rear brakes act a bit sooner).
At some point (1971?) GM rolled the prop valve, distribution block , hold-off valve, etc. all into a one-piece combo valve, but I think "everything" with front discs and rear drums in the 67-70 range had these.