The first one (the combination valve) was used on most GM disc applications from the mid-70's on, but there were at least 20 different part numbers due to unique calibrations for different car lines, weights, and engine options for different disc/drum (with an RPV in the drum outlet) and disc/disc (with no RPV at all) applications. Who knows how this one is calibrated, and it's not adjustable - if it isn't right for your combination, you have to replace it with another one until you find the right calibration.
The second one is quite common - Wilwood and several others make these, and they're easy to calibrate/adjust yourself to make sure the fronts always lock up first.