THis is what the Combination valve looked like on my 76 CJ with 4 wheel Drums:
Look a lot like a Disk/Disk one doesn't it.
It has the proportioning valve to the rear to prevent locking up the rears before the front because under hard breaking, the weight will transfer to the front and with less load on the back the rears will lock up and you loose control. It also hase a safety valve that will block off the compramised side of the brake system incase of failure which allow you to use the remaining system to try to stop.
If yours looks like this:
Then I would suspect it was set up for disk/drum brakes. With the metering system going to the front it would make for some poor braking with drums.
These Combination valves are setup like this with a metering valve on the front for the disk brakes and a proportioning valve on the back.
There is also something special about the MC for Drum/Drum setup. I think they have a 1 way valve iat each outlet to prevent the springs from pulling the pistons back to far into the slave cylinder. Otherwise you would have to pump up the brakes each time to use it.
And yeah #2 was the way my brakes were run on my 76 drum/drum.
THe drum brakes on my 76 were fantastic and easily stopped my CJ5 on 33" wheels. Even got better with the flexible SS brake lines. They just sucked when they got wet. That's why I converted to a 79 Disk/Drum setup.