Yes, the "uncle jack" spacers will fit TR3-4 with front disc brakes. Jack Drews developed them for his TR4 race car. So will the uprated spindles (HP661).
I believe the RPV can get gummed up or even rusty, the one in the photo was both. Not too obvious, but there is actually some metal rusted away from the small piece near the big spring. Old DOT 3 brake fluid is surprisingly corrosive and this car was a "barn find".
I disabled mine because it also makes the front brakes drag just a bit. Not enough to hurt anything (probably), but enough to annoy me. IIRC, that would have been about 1975, and the TR3A I had then (more of a backyard find) had some interesting braking problems. I eventually figured out that one of the front suspension mounts was torn loose from the frame, but it took an embarrassingly long time to figure that out. (In my defense, three different professional shops couldn't figure it out either.) Under hard braking, it would turn left no matter how hard you tried to turn right.
The valve was actually deleted later on; the TR4A SPC shows it as "Special order only" but I've heard that later TR4 didn't have it as well.
No idea why they are N/A; probably just not enough demand to make it worthwhile reproducing them.