1967 C20 Master Cylinder Plumbing

[sodly]

Hi. I'm a 60-66 guy contemplating updating my single-circuit master cylinder to a dual-circuit in my bone stock drum/drum, non-power-brake 1966 C20. I'm told the 1967 master cylinder is basically a direct swap. However, when I look at pics of stock 1967 and 1968 C20s I notice that the brake lines coming out of the master cylinder go immediately into some sort of proportioning block which I didn't think was necessary with drum/drum brakes. Can anyone explain what's going on here? What's the block for? Were C10s plumbed this way too? Or is the block just a C20 thing? Thanks.