What proportioning valve is this?

Actually, those look like '66 and earlier parts for the single pot master cylinder (which wasn't used in '67 and later). The distribution block is on the right, and the inline proportioning valve is on the left. I *believe* the small inline fitting is a residual valve, used in the rear line for the drums, since a single pot disc master wouldn't have one built in.
The distribution block in those early years only has four ports; one in from the master, one to each front wheel, and one to the rear (with the prop valve).
Very similar (if not the same) as the Mustang units from those years, which also used the K/H discs.
1667499106449.png
'67 and later systems with the dual-pot master cylinder used an all-in-one distribution/proportioning valve that looked more like this:
1667499957197.png