Correct Proportioning Valve?

Your part looks exactly like the B-body one according to my 1974 shop manual. The E-body one is very similar as well. hemi71x showed you the one for the A-body.

From the manual: "The metering valve holds off hydraulic pressure to the front disc brakes in the 3 to 135 psi range to allow the rear drum brake shoes to overcome the return springs and begin to contact the drums. This feature helps prevent locking the front brakes on icy surfaces under light braking conditions. The metering valve has no effect on front brake pressure during hard stops." Also, "A slight 'bump' can be felt by the foot as the brake pedal is stroked. This bump will occur after the pedal has been stroked about 1 inch." (If the bump is not felt, the valve is faulty.)

Otherwise as far as I can tell this valve has the same functionality as the A-body valve. According to the testing procedure, the rear pressure reduction is in the same range for both. It looks like the only difference is the metering function.