What proportioning valve is this?

Prof.,

Thanks! That would explain the mystery single reservoir master included with all the parts I got. I also got a rebuilt Napa dual reservoir master. But, I think it’s for a full drum setup as both reservoirs are the same size. Pics below.

So, are you saying that, theoretically, I could use the ‘66 roundish cylinder proportioning valve I have (pictured in original post) after my existing drum/drum distribution block for the rears?