I just purchased one from ebay for my 56 3100 chevy. It looks identical to that picture here. My came with two plugs. My dirrections says add your brake lines, you may use either side of the master cylinder, plug off the other side with the enclosed plugs. Oh the only difference my has a booster on it. this one you have should be for a drum /drum set up because if you had disks you would need a proportioning valve. It also says the lines closest to the booster is for the rear and farthest away from the booster is for the front and the lines do cross each other, because it need to be plumbed this way as the master cylinder ports are staggered to get the front braked to hit first creating the dive you need.
good luck
"As I lay rubber down the street, I pray for traction I can keep, but if I spin and begin to slide, please dear God protect my sweet ride." -Amen
56 Chevy 3100 
