What is this?

340 RR said
Sorry about that, don't know how I didn't include it. 1973 roadrunner with a 340. I'll try to get it off tomorrow and order a replacement mopar unit. I'm dreading going under the car to change the brake line but I'm sure that's gotta get replaced too.

JDMopar provided the first correct answer. While its true the automobile manufactures sometimes bought the same products from the brake manufactures, so that valve may have GM/Ford/AMC/IH applications too. My point is the metering valves were used on the mid size Chrysler products.

JDMopar said
Your proportioning valve is what would have come on your Roadrunner. I've had plenty of 72-73 Challengers and Cudas with that exact same valve. You can clean it out and rebuild it. I've got one just like it on my 71 Challenger conv. That car will stop on a dime and give you 9 cents change!

Metering valves hold off the initial pressure to the front brakes to reduce front wheel lock up on ice, etc.
MTSC 70-9 Brake Hydraulics 1970 Chrysler Imperial Hydraulic Brake Service Guide from the Master Technicians Service Conference Session 274

1972 Brake System Hydraulic Controls fo B-bodies

snip from
MTSC 72-4 Disk Brake Service Roundup, page 13
https://www.web.imperialclub.info/Repair/Lit/Master/293/page13.htm
Regardless, since the problem is with the rear brakes, the problem is not the metering valve but something blocking the passages to the rear. It could be in the block or anywhere between the master and the wheel cylinder.