Difference in Distribution Blocks

As mentioned, your old one is just a distribution block (tee) w/ imbalance warning switch. It came on drum-drum cars ~67+. Early front disk cars also used it, but with a separate prop valve downstream on the rear tube.

The new one is a "combination block", which moves the prop valve into the distribution block. It is only for disk-drum cars. You wouldn't get enough rear braking if you used it on a drum-drum car. However, you would barely notice since the rears provide only 30% of the braking anyway. Might install it now on a drum-drum if you plan to later change to front disks. I prefer a separate adjustable prop valve which is cheaper and better anyway.