I have a spare engine sitting and it has a Mazdatrix adjustable rear oil pressure regulator.
How do you adjust it?
Turn the nut, and then turn the body?
Closer the body to the nut, the more pressure? Or is it less?

 

Based on the physics of spring force... I would assume that the more compressed the spring is, the higher the relief pressure (spring constant "k" times the compression distance "x" = the force provided by the spring)... So basically, the more compressed the spring is, the more oil pressure you'll need to open the relief valve.
BUT, I would probably call them to double check... I'm an engineering student, not a proffesional engineer
***Madzdatrix 562-426-7960***

 

I would think you take the top nut off, insert shims, then put the nut back on. The regulator is similar to the older 13B regulators from the factory, which are multi piece and allow shimming.

Easy to shim your S4/S5/12A regulator otherwise:
http://www.aaroncake.net/rx-7/shimoil.htm