That master cyl was used as Front/rear split system here in OZ...we never got the diag split system at all, only the impot cars will have that crap
so same as said before, the front is the top, the rear is the bottom,,,rear is larger than the top is smaller
the person who owned the car before you probly got the sh!ts with the valve & removed it, no real drams there tho, they can be a pain in the butt anyways. They activate a valve & shut off one side of the system (front or rear) depending on which end it senses fluid moving faster than the other end & "Thinks" that it`s lost a cyl seal or split a hose etc & shuts off that end of the system,,,"Failiure consious" valve my dad calls em.
maybe just hook it up like it was & see what happens , if it was as good as you say then sweet

I know lots of guys get rid of the valve anyways & just size all the cyls accordingly,,,mainly fitting the smaller ("S" type) rear wheel cyls to save the rears locking up.
The earlier "siingle" system had a "pressure limiting" valve in the back subframe to assist with making less pressure to the rear for the same reason. we make them adjustable for the race cars for dry & wet/dirt reacing you will want different rear poressure yeah? know what i mean?
easy-peasy