So, I first used a Nitrile/Buna-N o-ring. Having heard from APR of the material for the quad-ring, I ordered the size-012 VITON quad-rings. I then, also checked the Nitrile ring and it was falling apart/shearing at the ID point of contact with the piston. At the same time I swapped it out for a VITON/FKM o-ring to use until the ordered quad-rings arrived. When removing the VITON o-ring for the quad-ring...I noted the VITON o-ring was still in great shape after over a week of use.
I cleaned the retainer nut and inspected the inner "pressed-in" shaft seal and decided, because it appears to be intact, not to measure it right now because I would have to remove it to get appropriate measurements with a caliper. What I will do in the future...is when needed, order a new hpfp w/Autotech internals. This will allow me to further dismantle the shaft seal and retainer nut of the APR internals. For now, I am done....